#include<iostream>
using namespace std;
int main(){
	int n;
	cin>>n;
	string s;
	cin>>s;
	for(int i=s.size()-1;i>=0;i--)
		if(s[i]=='A'||s[i]=='G'||s[i]=='C'||s[i]=='T')
			cout<<i+1<<endl,exit(0);
	cout<<0<<endl;
}