#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 ans=0;
  int up=n;
  int dw=-1;
  while(up-dw>2){
    int md1=(up+up+dw)/3,md2=(up+dw+dw)/3;
    ll nw1=0,nw2=0;
    {
      int ca=0;
      rep(i,n){
        if(s.at(i)=='?'){
          if(i<md1) ca++;
          else nw1+=ca;
        }
        if(s.at(i)=='A') ca++;
        if(s.at(i)=='C') nw1+=ca;
      }
    }
    {
      int ca=0;
      rep(i,n){
        if(s.at(i)=='?'){
          if(i<md2) ca++;
          else nw2+=ca;
        }
        if(s.at(i)=='A') ca++;
        if(s.at(i)=='C') nw2+=ca;
      }
    }
    if(nw1<nw2) up=md1;
    else dw=md2;
    ans=max(nw1,nw2);
  }
  cout<<ans<<endl;
}