/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2022.05.20 21:34:05
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;


int main() {
    int n;cin >> n;
    string s;cin >> s;
    for (int i = n-1; i >= 0; i--) {
        if (s[i] == 'A' || s[i] == 'G' || s[i] == 'C' || s[i] == 'T') {
            cout << i+1 << endl;
            return 0;
        }
    }
    cout << 0 << endl;
    return 0;
}