#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using ll=long long;
using Graph=vector<vector<int>>;
#define INF 1000000000
#define MOD 998244353
#define MAX 1000000

int main(){
  int n;
  cin>>n;
  string s;
  cin>>s;

  int left=n;
  int right=-1;
  int cnt=0;
  for(int i=0;i<n;i++){
    if(s[i]=='A'||s[i]=='G'||s[i]=='C'||s[i]=='T'){
      cnt++;
      left=min(left,i);
      right=max(right,i);
    }
  }

  cout<<right+1<<'\n';
}