#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
using namespace std;

using ll = long long;
const int INF = 1e9;
const ll inf = 1LL<<60;

void solve() {
  int n; cin >> n;
  string s; cin >> s;
  int ans = 0;
  for (int i=0; i<n; i++) {
    if (s[i] == 'A' || s[i] == 'G' || s[i] == 'C' || s[i] == 'T') ans = i+1;
  }
  cout << ans << '\n';
}

int main() {
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  // int t; cin >> t;
  /*while (t--)*/ solve();
}