#include <bits/stdc++.h>
using namespace std;

#ifdef _RUTHEN
#include "debug.hpp"
#else
#define show(...) true
#endif

using ll = long long;
#define rep(i, n) for (int i = 0; i < (n); i++)
template <class T> using V = vector<T>;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int N;
    cin >> N;
    string S;
    cin >> S;
    int ind = 0;
    rep(i, S.size()) {
        char c = S[i];
        if (c == 'A' || c == 'G' || c == 'C' || c == 'T') {
            ind = i + 1;
        }
    }
    cout << ind << '\n';
    return 0;
}