#include <bits/stdc++.h>
using namespace std;
int main(){
  int N;
  cin >> N;
  string S;
  cin >> S;
  vector<int> A(N + 1, 0), C(N + 1, 0);
  for (int i = 0; i < N; i++){
    A[i + 1] = A[i];
    C[i + 1] = C[i];
    if (S[i] == 'A'){
      A[i + 1]++;
    }
    if (S[i] == 'C'){
      C[i + 1]++;
    }
  }
  long long x = 0;
  for (int i = 0; i < N; i++){
    if (S[i] == 'C'){
      x += A[i];
    }
  }
  vector<long long> X, Y;
  int M = 0;
  for (int i = 0; i < N; i++){
    if (S[i] == '?'){
      M++;
      X.push_back(C[N] - C[i]);
      Y.push_back(A[i]);
    }
  }
  long long sumy = 0;
  for (int i = 0; i < M; i++){
    sumy += Y[i];
  }
  long long mx = sumy;
  for (int i = 0; i < M; i++){
    sumy -= Y[i];
    sumy += X[i];
    mx = max(mx, sumy + (long long) (i + 1) * (M - 1 - i));
  }
  cout << mx + x << endl;
}