#include<iostream>
#include<vector>
#include<cassert>
#define rep(i,n) for(i=0;i<(int)(n);i++)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;

int n;
string s;

int main(){
    int i,j;
    ll ans=0,tmp;
    scanf("%d",&n);
    cin>>s;
    vector<int> sumA(1,0),sumC(1,0),sumQ(1,0),qs;
    rep(i,n){
        sumA.push_back(sumA[i]+(s[i]=='A'));
        sumC.push_back(sumC[i]+(s[i]=='C'));
        sumQ.push_back(sumQ[i]+(s[i]=='?'));
        if(s[i]=='?')qs.push_back(i);
    }
    // ?の部分は，A~AC~Cのように選ぶのが最適
    // ?が全てCの場合から始める
    tmp=0;
    rep(i,n)if(s[i]=='A')tmp+=sumC[n]-sumC[i]+sumQ[n]-sumQ[i];
    ans=tmp;
    rep(i,qs.size()){
        tmp-=sumA[qs[i]]+i;
        tmp+=sumC[n]-sumC[qs[i]]+sumQ[n]-sumQ[qs[i]+1];
        ans=max(ans,tmp);
    }
    printf("%lld\n",ans);
    return 0;
}