#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <climits>
#include <map>
#include <queue>
#include <set>
#include <cstring>
#include <vector>

using namespace std;
typedef long long ll;
int N;
string S;

ll f(int x) {
  string T = S;
  int a_cnt = x;

  for (int i = 0; i < N; ++i) {
    if (S[i] != '?') continue;

    if (a_cnt > 0) {
      T[i] = 'A';
      --a_cnt;
    } else {
      T[i] = 'C';
    }
  }

  ll res = 0;
  ll cnt = 0;

  for (int i = 0; i < N; ++i) {
    if (T[i] == 'A') {
      ++cnt;
    } else {
      res += cnt;
    }
  }

  return res;
}

int main() {
  cin >> N;
  cin >> S;
  int A = 0;

  for (int i = 0; i < N; ++i) {
    if (S[i] == '?') {
      A++;
    }
  }

  int left = 0;
  int right = A;
  int loop_cnt = 100;

  for (int i = 0; i < loop_cnt; ++i) {
    int x = (2 * left + right) / 3;
    int y = (left + 2 * right) / 3;

    if (f(x) > f(y)) {
      right = y;
    } else {
      left = x;
    }
  }

  int base = (left + right) / 2;
  ll ans = 0;

  for (int x = max(0, base - 10); x <= min(A, base + 10); ++x) {
    ans = max(ans, f(x));
  }

  cout << ans << endl;

  return 0;
}