using System;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var n = int.Parse(Console.ReadLine());
        var s = Console.ReadLine().ToList();

        var index = s.FindLastIndex(x => x == 'A' || x == 'C' || x == 'G' || x == 'T');

        Console.WriteLine(index+1);
    }
}