//g++ 2.cpp -std=c++14 -O2 -I .
#include <bits/stdc++.h>
using namespace std;

#include <atcoder/all>
using namespace atcoder;

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

using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vld = vector<ld>;
using vvld = vector<vld>;
using vst = vector<string>;
using vvst = vector<vst>;

#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define pq_big(T) priority_queue<T,vector<T>,less<T>>
#define pq_small(T) priority_queue<T,vector<T>,greater<T>>
#define all(a) a.begin(),a.end()
#define rep(i,start,end) for(ll i=start;i<(ll)(end);i++)
#define per(i,start,end) for(ll i=start;i>=(ll)(end);i--)
#define uniq(a) sort(all(a));a.erase(unique(all(a)),a.end())

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

  int n;
  string s;
  cin>>n>>s;

  vi id;
  id.emplace_back(0);
  rep(i,0,n){
    if(s[i]=='A'||s[i]=='C'||s[i]=='G'||s[i]=='T'){
      id.emplace_back(i+1);
    }
  }

  cout<<id[id.size()-1]<<endl;
}