#include <bits/stdc++.h>

#include <limits>
#include <type_traits>

namespace suisen {
// ! utility
template <typename ...Types>
using constraints_t = std::enable_if_t<std::conjunction_v<Types...>, std::nullptr_t>;
template <bool cond_v, typename Then, typename OrElse>
constexpr decltype(auto) constexpr_if(Then&& then, OrElse&& or_else) {
    if constexpr (cond_v) {
        return std::forward<Then>(then);
    } else {
        return std::forward<OrElse>(or_else);
    }
}

// ! function
template <typename ReturnType, typename Callable, typename ...Args>
using is_same_as_invoke_result = std::is_same<std::invoke_result_t<Callable, Args...>, ReturnType>;
template <typename F, typename T>
using is_uni_op = is_same_as_invoke_result<T, F, T>;
template <typename F, typename T>
using is_bin_op = is_same_as_invoke_result<T, F, T, T>;

template <typename Comparator, typename T>
using is_comparator = std::is_same<std::invoke_result_t<Comparator, T, T>, bool>;

// ! integral
template <typename T, typename = constraints_t<std::is_integral<T>>>
constexpr int bit_num = std::numeric_limits<std::make_unsigned_t<T>>::digits;
template <typename T, unsigned int n>
struct is_nbit { static constexpr bool value = bit_num<T> == n; };
template <typename T, unsigned int n>
static constexpr bool is_nbit_v = is_nbit<T, n>::value;

// ?
template <typename T>
struct safely_multipliable {};
template <>
struct safely_multipliable<int> { using type = long long; };
template <>
struct safely_multipliable<long long> { using type = __int128_t; };
template <>
struct safely_multipliable<unsigned int> { using type = unsigned long long; };
template <>
struct safely_multipliable<unsigned long int> { using type = __uint128_t; };
template <>
struct safely_multipliable<unsigned long long> { using type = __uint128_t; };
template <>
struct safely_multipliable<float> { using type = float; };
template <>
struct safely_multipliable<double> { using type = double; };
template <>
struct safely_multipliable<long double> { using type = long double; };
template <typename T>
using safely_multipliable_t = typename safely_multipliable<T>::type;

} // namespace suisen

// ! type aliases
using i128 = __int128_t;
using u128 = __uint128_t;

template <typename T>
using pq_greater = std::priority_queue<T, std::vector<T>, std::greater<T>>;
template <typename T, typename U>
using umap = std::unordered_map<T, U>;

// ! macros (capital: internal macro)
#define OVERLOAD2(_1,_2,name,...) name
#define OVERLOAD3(_1,_2,_3,name,...) name
#define OVERLOAD4(_1,_2,_3,_4,name,...) name

#define REP4(i,l,r,s)  for(std::remove_reference_t<std::remove_const_t<decltype(r)>>i=(l);i<(r);i+=(s))
#define REP3(i,l,r)    REP4(i,l,r,1)
#define REP2(i,n)      REP3(i,0,n)
#define REPINF3(i,l,s) for(std::remove_reference_t<std::remove_const_t<decltype(l)>>i=(l);;i+=(s))
#define REPINF2(i,l)   REPINF3(i,l,1)
#define REPINF1(i)     REPINF2(i,0)
#define RREP4(i,l,r,s) for(std::remove_reference_t<std::remove_const_t<decltype(r)>>i=(l)+fld((r)-(l)-1,s)*(s);i>=(l);i-=(s))
#define RREP3(i,l,r)   RREP4(i,l,r,1)
#define RREP2(i,n)     RREP3(i,0,n)

#define rep(...)    OVERLOAD4(__VA_ARGS__, REP4   , REP3   , REP2   )(__VA_ARGS__)
#define rrep(...)   OVERLOAD4(__VA_ARGS__, RREP4  , RREP3  , RREP2  )(__VA_ARGS__)
#define repinf(...) OVERLOAD3(__VA_ARGS__, REPINF3, REPINF2, REPINF1)(__VA_ARGS__)

#define CAT_I(a, b) a##b
#define CAT(a, b) CAT_I(a, b)
#define UNIQVAR(tag) CAT(tag, __LINE__)
#define loop(n) for (std::remove_reference_t<std::remove_const_t<decltype(n)>> UNIQVAR(loop_variable) = n; UNIQVAR(loop_variable) --> 0;)

#define all(iterable) std::begin(iterable), std::end(iterable)
#define input(type, ...) type __VA_ARGS__; read(__VA_ARGS__)

#ifdef LOCAL
#  define debug(...) debug_internal(#__VA_ARGS__, __VA_ARGS__)

template <class T, class... Args>
void debug_internal(const char* s, T&& first, Args&&... args) {
    constexpr const char* prefix = "[\033[32mDEBUG\033[m] ";
    constexpr const char* open_brakets = sizeof...(args) == 0 ? "" : "(";
    constexpr const char* close_brakets = sizeof...(args) == 0 ? "" : ")";
    std::cerr << prefix << open_brakets << s << close_brakets << ": " << open_brakets << std::forward<T>(first);
    ((std::cerr << ", " << std::forward<Args>(args)), ...);
    std::cerr << close_brakets << "\n";
}

#else
#  define debug(...) void(0)
#endif

// ! I/O utilities

// pair
template <typename T, typename U>
std::ostream& operator<<(std::ostream& out, const std::pair<T, U> &a) {
    return out << a.first << ' ' << a.second;
}
// tuple
template <unsigned int N = 0, typename ...Args>
std::ostream& operator<<(std::ostream& out, const std::tuple<Args...> &a) {
    if constexpr (N >= std::tuple_size_v<std::tuple<Args...>>) {
        return out;
    } else {
        out << std::get<N>(a);
        if constexpr (N + 1 < std::tuple_size_v<std::tuple<Args...>>) {
            out << ' ';
        }
        return operator<<<N + 1>(out, a);
    }
}
// vector
template <typename T>
std::ostream& operator<<(std::ostream& out, const std::vector<T> &a) {
    for (auto it = a.begin(); it != a.end();) {
        out << *it;
        if (++it != a.end()) out << ' ';
    }
    return out;
}
// array
template <typename T, size_t N>
std::ostream& operator<<(std::ostream& out, const std::array<T, N> &a) {
    for (auto it = a.begin(); it != a.end();) {
        out << *it;
        if (++it != a.end()) out << ' ';
    }
    return out;
}
inline void print() { std::cout << '\n'; }
template <typename Head, typename... Tail>
inline void print(const Head &head, const Tail &...tails) {
    std::cout << head;
    if (sizeof...(tails)) std::cout << ' ';
    print(tails...);
}
template <typename Iterable>
auto print_all(const Iterable& v, std::string sep = " ", std::string end = "\n") -> decltype(std::cout << *v.begin(), void()) {
    for (auto it = v.begin(); it != v.end();) {
        std::cout << *it;
        if (++it != v.end()) std::cout << sep;
    }
    std::cout << end;
}

// pair
template <typename T, typename U>
std::istream& operator>>(std::istream& in, std::pair<T, U> &a) {
    return in >> a.first >> a.second;
}
// tuple
template <unsigned int N = 0, typename ...Args>
std::istream& operator>>(std::istream& in, std::tuple<Args...> &a) {
    if constexpr (N >= std::tuple_size_v<std::tuple<Args...>>) {
        return in;
    } else {
        return operator>><N + 1>(in >> std::get<N>(a), a);
    }
}
// vector
template <typename T>
std::istream& operator>>(std::istream& in, std::vector<T> &a) {
    for (auto it = a.begin(); it != a.end(); ++it) in >> *it;
    return in;
}
// array
template <typename T, size_t N>
std::istream& operator>>(std::istream& in, std::array<T, N> &a) {
    for (auto it = a.begin(); it != a.end(); ++it) in >> *it;
    return in;
}
template <typename ...Args>
void read(Args &...args) {
    ( std::cin >> ... >> args );
}

// ! integral utilities

// Returns pow(-1, n)
template <typename T>
constexpr inline int pow_m1(T n) {
    return -(n & 1) | 1;
}
// Returns pow(-1, n)
template <>
constexpr inline int pow_m1<bool>(bool n) {
    return -int(n) | 1;
}

// Returns floor(x / y)
template <typename T>
constexpr inline T fld(const T x, const T y) {
    return (x ^ y) >= 0 ? x / y : (x - (y + pow_m1(y >= 0))) / y;
}
template <typename T>
constexpr inline T cld(const T x, const T y) {
    return (x ^ y) <= 0 ? x / y : (x + (y + pow_m1(y >= 0))) / y;
}

template <typename T, suisen::constraints_t<suisen::is_nbit<T, 16>> = nullptr>
constexpr inline int popcount(const T x) { return __builtin_popcount(x); }
template <typename T, suisen::constraints_t<suisen::is_nbit<T, 32>> = nullptr>
constexpr inline int popcount(const T x) { return __builtin_popcount(x); }
template <typename T, suisen::constraints_t<suisen::is_nbit<T, 64>> = nullptr>
constexpr inline int popcount(const T x) { return __builtin_popcountll(x); }
template <typename T, suisen::constraints_t<suisen::is_nbit<T, 16>> = nullptr>
constexpr inline int count_lz(const T x) { return x ? __builtin_clz(x)   : suisen::bit_num<T>; }
template <typename T, suisen::constraints_t<suisen::is_nbit<T, 32>> = nullptr>
constexpr inline int count_lz(const T x) { return x ? __builtin_clz(x)   : suisen::bit_num<T>; }
template <typename T, suisen::constraints_t<suisen::is_nbit<T, 64>> = nullptr>
constexpr inline int count_lz(const T x) { return x ? __builtin_clzll(x) : suisen::bit_num<T>; }
template <typename T, suisen::constraints_t<suisen::is_nbit<T, 16>> = nullptr>
constexpr inline int count_tz(const T x) { return x ? __builtin_ctz(x)   : suisen::bit_num<T>; }
template <typename T, suisen::constraints_t<suisen::is_nbit<T, 32>> = nullptr>
constexpr inline int count_tz(const T x) { return x ? __builtin_ctz(x)   : suisen::bit_num<T>; }
template <typename T, suisen::constraints_t<suisen::is_nbit<T, 64>> = nullptr>
constexpr inline int count_tz(const T x) { return x ? __builtin_ctzll(x) : suisen::bit_num<T>; }
template <typename T>
constexpr inline int floor_log2(const T x) { return suisen::bit_num<T> - 1 - count_lz(x); }
template <typename T>
constexpr inline int ceil_log2(const T x)  { return floor_log2(x) + ((x & -x) != x); }
template <typename T>
constexpr inline int kth_bit(const T x, const unsigned int k) { return (x >> k) & 1; }
template <typename T>
constexpr inline int parity(const T x) { return popcount(x) & 1; }

struct all_subset {
    struct all_subset_iter {
        const int s; int t;
        constexpr all_subset_iter(int s) : s(s), t(s + 1) {}
        constexpr auto operator*() const { return t; }
        constexpr auto operator++() {}
        constexpr auto operator!=(std::nullptr_t) { return t ? (--t &= s, true) : false; }
    };
    int s;
    constexpr all_subset(int s) : s(s) {}
    constexpr auto begin() { return all_subset_iter(s); }
    constexpr auto end()   { return nullptr; }
};

// ! container

template <typename T, typename Comparator, suisen::constraints_t<suisen::is_comparator<Comparator, T>> = nullptr>
auto priqueue_comp(const Comparator comparator) {
    return std::priority_queue<T, std::vector<T>, Comparator>(comparator);
}

template <typename Iterable>
auto isize(const Iterable &iterable) -> decltype(int(iterable.size())) {
    return iterable.size();
}

template <typename T, typename Gen, suisen::constraints_t<suisen::is_same_as_invoke_result<T, Gen, int>> = nullptr>
auto generate_vector(int n, Gen generator) {
    std::vector<T> v(n);
    for (int i = 0; i < n; ++i) v[i] = generator(i);
    return v;
}
template <typename T>
auto generate_range_vector(T l, T r) {
    return generate_vector(r - l, [l](int i) { return l + i; });
}
template <typename T>
auto generate_range_vector(T n) {
    return generate_range_vector(0, n);
}

template <typename T>
void sort_unique_erase(std::vector<T> &a) {
    std::sort(a.begin(), a.end());
    a.erase(std::unique(a.begin(), a.end()), a.end());
}

template <typename InputIterator, typename BiConsumer>
auto foreach_adjacent_values(InputIterator first, InputIterator last, BiConsumer f) -> decltype(f(*first++, *last), void()) {
    if (first != last) for (auto itr = first, itl = itr++; itr != last; itl = itr++) f(*itl, *itr);
}
template <typename Container, typename BiConsumer>
auto foreach_adjacent_values(Container c, BiConsumer f) -> decltype(c.begin(), c.end(), void()){
    foreach_adjacent_values(c.begin(), c.end(), f);
}

// ! other utilities

// x <- min(x, y). returns true iff `x` has chenged.
template <typename T>
inline bool chmin(T &x, const T &y) {
    if (y >= x) return false;
    x = y;
    return true;
}
// x <- max(x, y). returns true iff `x` has chenged.
template <typename T>
inline bool chmax(T &x, const T &y) {
    if (y <= x) return false;
    x = y;
    return true;
}

namespace suisen {}
using namespace suisen;
using namespace std;

struct io_setup {
    io_setup(int precision = 20) {
        std::ios::sync_with_stdio(false);
        std::cin.tie(nullptr);
        std::cout << std::fixed << std::setprecision(precision);
    }
} io_setup_ {};

// ! code from here

#include <cassert>
#include <sstream>
#include <string>
#include <tuple>

#include <deque>
#include <vector>

namespace suisen {
    template <typename T, bool auto_extend = false>
    struct ObjectPool {
        using value_type = T;
        using value_pointer_type = T*;

        template <typename U>
        using container_type = std::conditional_t<auto_extend, std::deque<U>, std::vector<U>>;

        container_type<value_type> pool;
        container_type<value_pointer_type> stock;
        decltype(stock.begin()) it;

        ObjectPool() : ObjectPool(0) {}
        ObjectPool(int siz) : pool(siz), stock(siz) {
            clear();
        }

        int capacity() const { return pool.size(); }
        int size() const { return it - stock.begin(); }

        value_pointer_type alloc() {
            if constexpr (auto_extend) ensure();
            return *it++;
        }

        void free(value_pointer_type t) {
            *--it = t;
        }

        void clear() {
            int siz = pool.size();
            it = stock.begin();
            for (int i = 0; i < siz; i++) stock[i] = &pool[i];
        }

        void ensure() {
            if (it != stock.end()) return;
            int siz = stock.size();
            for (int i = siz; i <= siz * 2; ++i) {
                stock.push_back(&pool.emplace_back());
            }
            it = stock.begin() + siz;
        }
    };
} // namespace suisen

namespace suisen::bbst::internal {
    template <typename T, typename Derived>
    struct RedBlackTreeNodeBase {
        enum RedBlackTreeNodeColor { RED, BLACK };

        using base_type = void;
        using size_type = int;

        using value_type = T;

        using node_type = Derived;
        using tree_type = node_type*;

        using color_type = RedBlackTreeNodeColor;

        RedBlackTreeNodeBase() = default;

        static inline ObjectPool<node_type> pool{};

        static void init_pool(int siz) { pool = ObjectPool<node_type>(siz); }
        static int node_num() { return pool.size(); }

        static tree_type empty_tree() { return nullptr; }

        static size_type size(tree_type node) { return node ? node->_siz : 0; }
        static bool empty(tree_type node) { return not node; }

        template <bool force_black_root = true>
        static tree_type merge(tree_type l, tree_type r) {
            if (not l) return r;
            if (not r) return l;

            tree_type res = nullptr;
            if (size_type hl = height(l), hr = height(r); hl > hr) {
                l = node_type::push(l);
                tree_type c = l->_ch[1] = merge<false>(l->_ch[1], r);
                if (l->_col == BLACK and c->_col == RED and color(c->_ch[1]) == RED) {
                    std::swap(l->_col, c->_col);
                    if (std::exchange(l->_ch[0]->_col, BLACK) == BLACK) return rotate(l, 1);
                }
                res = node_type::update(l);
            } else if (hr > hl) {
                r = node_type::push(r);
                tree_type c = r->_ch[0] = merge<false>(l, r->_ch[0]);
                if (r->_col == BLACK and c->_col == RED and color(c->_ch[0]) == RED) {
                    std::swap(r->_col, c->_col);
                    if (std::exchange(r->_ch[1]->_col, BLACK) == BLACK) return rotate(r, 0);
                }
                res = node_type::update(r);
            } else {
                res = create_branch(l, r);
            }
            if constexpr (force_black_root) res->_col = BLACK;
            return res;
        }

        static std::pair<tree_type, tree_type> split(tree_type node, size_type k) {
            if (not node) return { nullptr, nullptr };
            node = node_type::push(node);
            if (k == 0) return { nullptr, node };
            if (k == size(node)) return { node, nullptr };

            tree_type l = std::exchange(node->_ch[0], nullptr);
            tree_type r = std::exchange(node->_ch[1], nullptr);

            free_node(node);

            if (color(l) == RED) l->_col = BLACK;
            if (color(r) == RED) r->_col = BLACK;

            size_type szl = size(l);
            tree_type m;
            if (k < szl) {
                std::tie(l, m) = split(l, k);
                return { l, merge(m, r) };
            }
            if (k > szl) {
                std::tie(m, r) = split(r, k - szl);
                return { merge(l, m), r };
            }
            return { l, r };
        }

        static std::tuple<tree_type, tree_type, tree_type> split_range(tree_type node, size_type l, size_type r) {
            auto [tlm, tr] = split(node, r);
            auto [tl, tm] = split(tlm, l);
            return { tl, tm, tr };
        }

        static tree_type insert(tree_type node, size_type k, const value_type& val) {
            auto [tl, tr] = split(node, k);
            return merge(merge(tl, create_leaf(val)), tr);
        }
        static tree_type push_front(tree_type node, const value_type &val) { return insert(node, 0, val); }
        static tree_type push_back(tree_type node, const value_type &val) { return insert(node, size(node), val); }

        static std::pair<tree_type, value_type> erase(tree_type node, size_type k) {
            auto [tl, tm, tr] = split_range(node, k, k + 1);
            value_type erased_value = tm->_val;
            free_node(tm);
            return { merge(tl, tr) , erased_value };
        }
        static std::pair<tree_type, value_type> pop_front(tree_type node) { return erase(node, 0); }
        static std::pair<tree_type, value_type> pop_back(tree_type node) { return erase(node, size(node) - 1); }

        template <typename U>
        static tree_type build(const std::vector<U>& a, int l, int r) {
            if (r - l == 1) return create_leaf(a[l]);
            int m = (l + r) >> 1;
            return merge(build(a, l, m), build(a, m, r));
        }
        template <typename U>
        static tree_type build(const std::vector<U>& a) {
            return a.empty() ? empty_tree() : build(a, 0, a.size());
        }

        template <typename OutputIterator>
        static void dump(tree_type node, OutputIterator it) {
            if (empty(node)) return;
            auto dfs = [&](auto dfs, tree_type cur) -> void {
                if (cur->is_leaf()) {
                    *it++ = cur->_val;
                    return;
                }
                dfs(dfs, cur->_ch[0]);
                dfs(dfs, cur->_ch[1]);
            };
            dfs(dfs, node);
        }

        // Don't use on persistent tree.
        static void free(tree_type node) {
            auto dfs = [&](auto dfs, tree_type cur) -> void {
                if (not cur) return;
                dfs(dfs, cur->_ch[0]);
                dfs(dfs, cur->_ch[1]);
                free_node(cur);
            };
            dfs(dfs, node);
        }

        template <typename ToStr>
        static std::string to_string(tree_type node, ToStr f) {
            std::vector<value_type> dat;
            node_type::dump(node, std::back_inserter(dat));
            std::ostringstream res;
            int siz = dat.size();
            res << '[';
            for (int i = 0; i < siz; ++i) {
                res << f(dat[i]);
                if (i != siz - 1) res << ", ";
            }
            res << ']';
            return res.str();
        }
        static std::string to_string(tree_type node) {
            return to_string(node, [](const auto &e) { return e; });
        }

        static void check_rbtree_properties(tree_type node) {
            assert(color(node) == BLACK);
            auto dfs = [&](auto dfs, tree_type cur) -> int {
                if (not cur) return 0;
                if (cur->_col == RED) {
                    assert(color(cur->_ch[0]) == BLACK);
                    assert(color(cur->_ch[1]) == BLACK);
                }
                int bl = dfs(dfs, cur->_ch[0]);
                int br = dfs(dfs, cur->_ch[1]);
                assert(bl == br);
                return bl + (cur->_col == BLACK);
            };
            dfs(dfs, node);
        }

    protected:
        color_type _col;
        tree_type _ch[2]{ nullptr, nullptr };
        value_type _val;
        size_type _siz, _lev;

        RedBlackTreeNodeBase(const value_type& val) : _col(BLACK), _val(val), _siz(1), _lev(0) {}
        RedBlackTreeNodeBase(tree_type l, tree_type r) : _col(RED), _ch{ l, r }, _siz(l->_siz + r->_siz), _lev(l->_lev + (l->_col == BLACK)) {}

        static void clear_pool() { pool.clear(); }
        static int pool_capacity() { return pool.capacity(); }

        static color_type color(tree_type node) { return node ? node->_col : BLACK; }
        static size_type height(tree_type node) { return node ? node->_lev : 0; }

        bool is_leaf() const { return not (_ch[0] or _ch[1]); }

        static tree_type clone(tree_type node) {
            return node;
        }
        static tree_type update(tree_type node) {
            node->_siz = node->is_leaf() ? 1 : size(node->_ch[0]) + size(node->_ch[1]);
            node->_lev = node->_ch[0] ? height(node->_ch[0]) + (node->_ch[0]->_col == BLACK) : 0;
            return node;
        }
        static tree_type push(tree_type node) {
            return node;
        }

        static tree_type rotate(tree_type node, int index) {
            node = node_type::push(node);
            tree_type ch_node = node_type::push(node->_ch[index]);
            node->_ch[index] = std::exchange(ch_node->_ch[index ^ 1], node);
            return node_type::update(node), node_type::update(ch_node);
        }

        static tree_type create_leaf(const value_type& val = value_type{}) {
            return &(*pool.alloc() = node_type(val));
        }

        static tree_type create_branch(tree_type l, tree_type r) {
            return node_type::update(&(*pool.alloc() = node_type(l, r)));
        }

        static void free_node(tree_type node) {
            if (node) pool.free(node);
        }
    };
} // namespace suisen

namespace suisen::bbst {
    template <typename T, template <typename, typename> typename BaseNode = internal::RedBlackTreeNodeBase>
    struct RedBlackTreeNode : public BaseNode<T, RedBlackTreeNode<T, BaseNode>> {
        using base_type = BaseNode<T, RedBlackTreeNode<T, BaseNode>>;
        using node_type = typename base_type::node_type;
        using tree_type = typename base_type::tree_type;
        using size_type = typename base_type::size_type;
        using value_type = typename base_type::value_type;

        friend base_type;
        friend typename base_type::base_type;

        RedBlackTreeNode() = default;

    private:
        RedBlackTreeNode(const value_type& val) : base_type(val) {}
        RedBlackTreeNode(tree_type l, tree_type r) : base_type(l, r) {}
    };
}

using Node = bbst::RedBlackTreeNode<char>;
using Tree = Node::tree_type;

constexpr int A = 'A' - 'A';
constexpr int G = 'G' - 'A';
constexpr int C = 'C' - 'A';
constexpr int T = 'T' - 'A';

int main() {
    Node::init_pool(6000000);

    input(int, n);
    vector<char> s(n);
    read(s);

    auto seq = Node::build(s);

    array<int, 26> cnt{};
    for (char c : s) {
        ++cnt[c - 'A'];
    }

    int offset = 0;

    auto get_index = [&](int ch) {
        int x = (ch - offset) % 26;
        if (x < 0) x += 26;
        return x;
    };

    int ans = 0;
    for (;; ++ans) {
        int num = cnt[get_index(A)] + cnt[get_index(G)] + cnt[get_index(C)] + cnt[get_index(T)];
        if (num == 0) break;
        
        char c;
        tie(seq, c) = Node::erase(seq, num - 1);

        int ch = (offset + c - 'A') % 26;
        int c2 = --cnt[get_index(ch)];
        offset += c2;
    }

    print(ans);

    return 0;
}