import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		char[] s = sc.next().toCharArray();
		sc.close();

		for (int i = n - 1; i >= 0; i--) {
			if (s[i] == 'A' || s[i] == 'G' || s[i] == 'C' || s[i] == 'T') {
				System.out.println(i + 1);
				return;
			}
		}
		System.out.println(0);
	}
}