#include <iostream>
#include <vector>
#include <cmath>
using namespace std;
typedef long long ll;

bool agct(char c){
	return c=='A' or c=='G' or c=='C' or c=='T';
}

int main() {
	int n;string s;cin >>n>>s;
	int ans=0;
	for(int i=n-1;i>=0;i--)if(agct(s[i])) {
		ans=i+1;
		break;
	}
	cout<<ans<<endl;
	return 0;
}