#include <iostream>
#include <cassert>
#include <set>

using namespace std;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  int n; cin >> n;
  string s; cin >> s;
  set<char> st = {'A','G','C','T'};
  int ma = 0;
  for (int i = 0; i < n; i++) {
    if (st.find(s[i]) != st.end()) ma = i + 1;
  }
  cout << ma << endl;
  return 0;
}