#include <bits/stdc++.h>
#include <atcoder/all>

using namespace std;

//#define DISABLE_PRINT

#if defined(ENABLE_PRINT) && !defined(DISABLE_PRINT)

#define P(...) fprintf(stderr, __VA_ARGS__)
#define P2(fmt) fprintf(stderr, fmt)
#define LP fprintf(stderr, "L: %d\n", __LINE__)

#else

#define P(...) ((void)0)
#define P2(fmt) ((void)0)
#define LP ((void)0)

#endif

#define rep(i, n) for(int i = 0; i < (int)(n); ++i)
#define ALL(x) x.begin(),x.end()

using ll = long long;
using ull = unsigned long long;

int main(int, const char**)
{
    int N; cin >> N;
    string S; cin >> S;
    vector<int> nexts(N + 1);
    rep(i, N) nexts[i] = i + 1;

    auto check = [&](int i) {
        assert(i < N);
        if(S[i] == 'A' || S[i] == 'G' || S[i] == 'C' || S[i] == 'T') return true;
        return false;
    };

    auto c = 0;
    rep(i, N) {
        if(check(i)) c++;
    }

    int ans = 0;
    auto count = c;
    P("count: %d\n", count);
    while(c > 0) {
        auto ti = c - 1;
        auto prev = ti - 1;
        while(ti < N) {
            P("c: %d, ti: %d\n", c, ti);
            ans++;
            if(check(ti)) {
                c--;
                if(prev >= 0) {
                    nexts[prev] = ti + 1;
                }
                break;
            }
            ti = nexts[ti];
        }
    }
    cout << ans << endl;
    return 0;
}