#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;
    };

    int c = -1;
    rep(i, N) {
        if(check(i)) c = i;
    }
    cout << c + 1 << endl;

    return 0;
}