import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int num = sc.nextInt();
		String[] strAry = (sc.next()).split("");
		
		boolean flag = false;
		int i = strAry.length - 1;
		for ( ; -1 < i ; i--) {
			switch(strAry[i]) {
				case "A":
				case "G":
				case "C":
				case "T":
					flag = true;
					break;
			}
			
			if (flag) {
				break;
			}
		}
		
		System.out.println(i + 1);
		
		sc.close();
		
	}
}