#include "bits/stdc++.h"
#include <random>
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define SZ(x) ((lint)(x).size())
#define FOR(i, begin, end) for(lint i=(begin),i##_end_=(end);i<i##_end_;++i)
#define IFOR(i, begin, end) for(lint i=(end)-1,i##_begin_=(begin);i>=i##_begin_;--i)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
#define endk '\n'
using namespace std; typedef unsigned long long _ulong; typedef long long int lint; typedef long double ld; typedef pair<lint, lint> plint; typedef pair<ld, ld> pld;
struct fast_ios { fast_ios() { cin.tie(nullptr), ios::sync_with_stdio(false), cout << fixed << setprecision(10); }; } fast_ios_;
template<class T> auto add = [](T a, T b) -> T { return a + b; };
template<class T> auto mul = [](T a, T b) -> T { return a * b; };
template<class T> auto f_max = [](T a, T b) -> T { return max(a, b); };
template<class T> auto f_min = [](T a, T b) -> T { return min(a, b); };
template<class T> using V = vector<T>;
using Vl = V<lint>; using VVl = V<Vl>; using VVVl = V<V<Vl>>;
template< typename T > ostream& operator<<(ostream& os, const vector< T >& v) {
    for (int i = 0; i < (int)v.size(); i++) os << v[i] << (i + 1 != v.size() ? " " : "");
    return os;
}
template< typename T >istream& operator>>(istream& is, vector< T >& v) {
    for (T& in : v) is >> in;
    return is;
}
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return 1; } return 0; }
template <class T>
T div_floor(T a, T b) {
    if (b < 0) a *= -1, b *= -1;
    return a >= 0 ? a / b : (a + 1) / b - 1;
}
template <class T>
T div_ceil(T a, T b) {
    if (b < 0) a *= -1, b *= -1;
    return a > 0 ? (a - 1) / b + 1 : a / b;
}
template <class F> struct rec {
    F f;
    rec(F&& f_) : f(std::forward<F>(f_)) {}
    template <class... Args> auto operator()(Args &&... args) const {
        return f(*this, std::forward<Args>(args)...);
    }
};
lint gcd(lint a, lint b) { if (b == 0) return a; else return gcd(b, a % b); }
lint digit(lint a) { return (lint)log10(a); }
lint e_dist(plint a, plint b) { return abs(a.first - b.first) * abs(a.first - b.first) + abs(a.second - b.second) * abs(a.second - b.second); }
lint m_dist(plint a, plint b) { return abs(a.first - b.first) + abs(a.second - b.second); }
bool check_overflow(lint a, lint b, lint limit) { if (b == 0) return false; return a >= limit / b; } // a * b > c => true
void Worshall_Floyd(VVl& g) { REP(k, SZ(g)) REP(i, SZ(g)) REP(j, SZ(g)) chmin(g[i][j], g[i][k] + g[k][j]); }
const lint MOD1000000007 = 1000000007, MOD998244353 = 998244353, INF = 4e18;
lint dx[8] = { 0, 1, 0, -1, 1, -1, 1, -1 }, dy[8] = { 1, 0, -1, 0, -1, -1, 1, 1 };
bool YN(bool flag) { cout << (flag ? "YES" : "NO") << endk; return flag; } bool yn(bool flag) { cout << (flag ? "Yes" : "No") << endk; return flag; }
struct Edge {
    lint from, to;
    lint cost, t;
    Edge() {

    }
    Edge(lint u, lint v, lint c, lint _t) {
        cost = c;
        from = u;
        to = v;
        t = _t;
    }
    bool operator<(const Edge& e) const {
        return cost < e.cost;
    }
};
struct WeightedEdge {
    lint to;
    lint cost;
    WeightedEdge(lint v, lint c) {
        to = v;
        cost = c;
    }
    bool operator<(const WeightedEdge& e) const {
        return cost < e.cost;
    }
};
using WeightedGraph = V<V<WeightedEdge>>;
typedef pair<lint, plint> tlint;
typedef pair<ld, ld> pld;
typedef pair<plint, plint> qlint;
typedef pair<string, string> pstr;
typedef pair<lint, Vl> valv;

struct BinaryIndexedTree {
    int n;
    int b;
    vector<lint> bit;
    BinaryIndexedTree(int n_) : n(n_ + 1), bit(n, 0) {}
    void add(int i, lint x) {
        for (int idx = i; idx < n; idx += (idx & -idx)) {
            bit[idx] += x;
        }
    }
    lint sum(int i) {
        lint s = 0;
        for (int idx = i; idx > 0; idx -= (idx & -idx)) {
            s += bit[idx];
        }
        return s;
    }
    int lower_bound(lint w) {
        if (w <= 0) {
            return 0;
        }
        else {
            int x = 0, r = 1;
            while (r < n) r = r << 1;
            for (int len = r; len > 0; len = len >> 1) {
                if (x + len < n && bit[x + len] < w) {
                    w -= bit[x + len];
                    x += len;
                }
            }
            return x;
        }
    }
    void show() {
        for (int i = 0; i < n - 1; i++) {
            cout << sum(i) - sum(i - 1) << " ";
        }
        cout << endk;
    }
};

struct ExtendBinaryIndexedTree {
public:
    ExtendBinaryIndexedTree(int _n) { init(_n); }
    //半開区間[l, r)
    void add(int l, int r, lint x) {
        add_sub(0, l, -x * (l - 1));
        add_sub(0, r, x * (r - 1));
        add_sub(1, l, x);
        add_sub(1, r, -x);
    }
    //区間[0, i)
    lint sum(int i) {
        return sum_sub(0, i) + sum_sub(1, i) * i;
    }

    void show() {
        for (int i = 0; i < n - 1; i++) {
            cout << sum(i + 1) - sum(i) << " ";
        }
        cout << endk;
    }
private:
    int n;
    vector<lint> bit[2];
    void init(int _n) {
        n = _n + 1;
        bit[0].assign(n, 0);
        bit[1].assign(n, 0);
    }
    void add_sub(int p, int i, lint x) {
        for (int idx = i; idx < n; idx += (idx & -idx)) {
            bit[p][idx] += x;
        }
    }
    lint sum_sub(int p, int i) {
        lint s = 0;
        for (int idx = i; idx > 0; idx -= (idx & -idx)) {
            s += bit[p][idx];
        }
        return s;
    }
};

int main() {
    lint N;
    cin >> N;
    string s;
    cin >> s;
    Vl cnt(26);
    BinaryIndexedTree bit(N);
    REP(i, N) bit.add(i + 1, 1);
    REP(i, N) cnt[s[i] - 'A']++;

    lint ans = 0, d = 0;
    while (true) {
        lint sum = 0;
        sum += cnt[(('A' - 'A' - d) % 26 + 26) % 26];
        sum += cnt[(('C' - 'A' - d) % 26 + 26) % 26];
        sum += cnt[(('T' - 'A' - d) % 26 + 26) % 26];
        sum += cnt[(('G' - 'A' - d) % 26 + 26) % 26];
        if (sum == 0) break;
        lint p = bit.lower_bound(sum);
        bit.add(p + 1, -1);
        cnt[s[p] - 'A']--;
        lint sum2 = cnt[s[p] - 'A'];
        d += sum2;
        ans++;
    }
    cout << ans << endk;
}