/* -*- 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 */

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

/* main */

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

  int x0 = 0, x1 = n;
  while (x0 + 2 < x1) {
    int xx0 = (x0 * 2 + x1) / 3;
    int xx1 = (x0 + x1 * 2) / 3;
    ll c0 = check(n, xx0);
    ll c1 = check(n, xx1);
    if (c0 < c1) x0 = xx0;
    else x1 = xx1;
  }

  ll maxc = 0;
  for (int x = x0; x <= x1; x++)
    maxc = max(maxc, check(n, x));

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

  return 0;
}