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