/* -*- coding: utf-8 -*-
 *
 * 1943.cc:  No.1943 消えたAGCT(1) - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_N = 500000;

/* typedef */

/* global variables */

bool cmap[128];
char s[MAX_N + 4];

/* subroutines */

/* main */

int main() {
  cmap['A'] = cmap['G'] = cmap['C'] = cmap['T'] = true;

  int n;
  scanf("%d%s", &n, s);

  int k = n - 1;
  while (k >= 0 && ! cmap[s[k]]) k--;

  printf("%d\n", k + 1);
  return 0;
}