/* -*- coding: utf-8 -*-
 *
 * 2276.cc:  No.2276 I Want AC - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_N = 200000;

/* typedef */

typedef long long ll;

/* global variables */

char s[MAX_N + 4];

/* subroutines */

/* main */

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

  ll sum = 0;
  int c = 0;
  for (int i = 0, a = 0; i < n; i++) {
    if (s[i] == 'A') a++;
    else sum += a, c++;
  }

  ll maxc = sum;
  for (int i = 0, a = 0; i < n; i++) {
    if (s[i] == '?') {
      sum -= a;
      a++, c--;
      sum += c;
      maxc = max(maxc, sum);
    }
    else if (s[i] == 'A') a++;
    else c--;
  }

  printf("%lld\n", maxc);

  return 0;
}