#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
//const long nPrime = 1000000007;
//const long nPrime = 998244353;
typedef long long ll;
int main() {
    ll n;
    cin >> n;
    string s;
    cin >> s;
    ll nAns = 0;
    for(ll i = 0; i < n; i++){
        if(s[i] == 'A' || s[i] == 'G' || s[i] == 'C' || s[i] == 'T'){
            nAns = i+1;
        }
    }
    cout << nAns << endl;
    return 0;
}