#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,n) for (int i=0;i<(int)(n);i++)

int main(){
  int n;
  cin>>n;
  string s;
  cin>>s;
  set<char> st={'A','G','C','T'};
  int cn=-1;
  
  rep(i,n) if(st.count(s.at(i))) cn=i;
  cout<<cn+1<<endl;
}