#include<stdio.h>
int main()
{
	int n;
	scanf("%d", &n);
	char s[500005];
	scanf("%s", s);
	int i, ans = 0;
	for (i = 0; i < n; i++)
		if (s[i] == 'A' || s[i] == 'T' || s[i] == 'G' || s[i] == 'C')
			ans = i + 1;
	printf("%d\n", ans);
	return 0;
}