#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main(){

    int N, ans=-1;
    string S;
    cin >> N >> S;
    for (int i=0; i<N; i++){
        if (S[i] == 'A' || S[i] == 'T' || S[i] == 'C' || S[i] == 'G') ans=i;
    }

    cout << ans+1 << endl;

    return 0;
}