#include <bits/stdc++.h>
using namespace std;
int main(){
  int n;
  cin >> n;
  string s;
  cin >> s;
  int mx = 0;
  for (int i = 0; i < n; i++){
    if (s[i] == 'A' || s[i] == 'G' || s[i] == 'C' || s[i] == 'T'){
      mx = i + 1;
    }
  }
  cout << mx << endl;
}