#line 1 "main.cpp"
#pragma region Macros
#include <bits/stdc++.h>
using namespace std;
template <class T> inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T> inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return 1;
    }
    return 0;
}
#ifdef DEBUG
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
    os << '(' << p.first << ',' << p.second << ')';
    return os;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
    os << '{';
    for(int i = 0; i < (int)v.size(); i++) {
        if(i) { os << ','; }
        os << v[i];
    }
    os << '}';
    return os;
}
void debugg() { cerr << endl; }
template <class T, class... Args>
void debugg(const T &x, const Args &... args) {
    cerr << " " << x;
    debugg(args...);
}
#define debug(...)                                                             \
    cerr << __LINE__ << " [" << #__VA_ARGS__ << "]: ", debugg(__VA_ARGS__)
#define dump(x) cerr << __LINE__ << " " << #x << " = " << (x) << endl
#else
#define debug(...) (void(0))
#define dump(x) (void(0))
#endif

struct Setup {
    Setup() {
        cin.tie(0);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(15);
    }
} __Setup;

using ll = long long;
#define OVERLOAD3(_1, _2, _3, name, ...) name
#define ALL(v) (v).begin(), (v).end()
#define RALL(v) (v).rbegin(), (v).rend()
#define REP1(i, n) for(int i = 0; i < int(n); i++)
#define REP2(i, a, b) for(int i = (a); i < int(b); i++)
#define REP(...) OVERLOAD3(__VA_ARGS__, REP2, REP1)(__VA_ARGS__)
#define UNIQUE(v) sort(ALL(v)), (v).erase(unique(ALL(v)), (v).end())
#define SZ(v) ((int)(v).size())
const int INF = 1 << 30;
const ll LLINF = 1LL << 60;
constexpr int MOD = 1000000007;
constexpr int MOD2 = 998244353;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

void Case(int i) { cout << "Case #" << i << ": "; }
int popcount(int x) { return __builtin_popcount(x); }
ll popcount(ll x) { return __builtin_popcountll(x); }
#pragma endregion Macros

#line 1 "/Users/siro53/kyo-pro/compro_library/data_structure/BIT.hpp"
template <typename T> struct BIT {
    int n;
    vector<T> bit;
    BIT(){}
    BIT(int _n) { init(_n); }
    void init(int _n) {
        n = _n + 1;
        bit.resize(n + 1, 0);
    }
    // [0, k)
    T sum(int k) {
        T res = 0;
        for(int i = k - 1; i >= 0; i = (i & (i + 1)) - 1) { res += bit[i]; }
        return res;
    }
    // [l, r)
    T sum(int l, int r) { return (l < r ? sum(r) - sum(l) : 0); }
    // bit[k] += x
    void add(int k, T x) {
        for(int i = k; i < n; i |= i + 1) { bit[i] += x; }
    }
    // v[0] + ... + v[res] >= x
    int lower_bound(T x) {
        int res = -1;
        int k = 1;
        while(2 * k <= n) { k <<= 1; }
        for(; k > 0; k >>= 1) {
            if(res + k < n && bit[res + k] < x) {
                x -= bit[res + k];
                res += k;
            }
        }
        return res + 1;
    }
};
#line 77 "main.cpp"
const string rna = "AGCT";

int main() {
    int N;
    string s;
    cin >> N >> s;

    vector<BIT<int>> bts(26, BIT<int>(N));
    REP(i, N) bts[s[i] - 'A'].add(i, 1);
    BIT<int> btall(N);
    REP(i, N) btall.add(i, 1);

    int offset = 0;
    int ans = 0;
    while(1) {
        int c1 = 0;
        for(char c : rna) c1 += bts[((c - 'A') + offset) % 26].sum(N);
        if(c1 == 0) break;
        int id = btall.lower_bound(c1);
        assert(id < N);
        btall.add(id, -1);
        int c2 = -1;
        REP(i, 26) if(bts[(i + offset) % 26].sum(id, id+1)) {
            bts[(i + offset) % 26].add(id, -1);
            c2 = bts[(i + offset) % 26].sum(N);
            break;
        } 
        assert(c2 != -1);
        offset += 2600000 - c2;
        offset %= 26;
        ans++;
    }

    cout << ans << endl;
}