#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,n) for (int i=0;i<(int)(n);i++)

int main(){
  int n;
  cin>>n;
  string s;
  cin>>s;
  ll nw=0;
  int ca=0;
  rep(i,n){
    if(s.at(i)=='A') ca++;
    else nw+=ca;
  }
  int cc=n-ca;
  ll ans=nw;
  ca=0;
  rep(i,n){
    if(s.at(i)=='A') ca++;
    if(s.at(i)=='C') cc--;
    if(s.at(i)=='?'){
      cc--;
      nw-=ca;
      nw+=cc;
      ans=max(ans,nw);
      ca++;
    }
  }
  cout<<ans<<endl;
}