#include<bits/stdc++.h>

using namespace std;

using ll = long long;
using ld = long double;

#define rep(i, a, b) for(ll i = (ll)a; i < (ll)b; i++)
#define irep(i, v) for(auto i = (v).begin(); i != (v).end(); i++)
#define ALL(v) (v).begin(), (v).end()
#define SZ(v) (ll)(v).size()

const ll INF = 1e18;
const ld EPS = 1e-10;
const ll MOD = 1e9 + 7;
const ld PI = M_PI;

int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  srand((unsigned)time(NULL));
  cout << fixed << setprecision(20);

  ll n;
  string s;
  cin >> n >> s;
  ll ans = 0;
  rep(i, 0, n){
    if(s[i] == 'A' || s[i] == 'G' || s[i] == 'C' || s[i] == 'T'){
      ans = i + 1;
    }
  }

  cout << ans << endl;


  return 0;
}