#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)(s); i < (int)(e); ++i)
#define all(a) (a).begin(),(a).end()

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