#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
using ll=long long;
#define rep(i,n) for(int i=0;i<n;i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
template<class T> bool chmax(T &a, T b){if (a < b){a = b;return true;} else return false;}
template<class T> bool chmin(T &a, T b){if (a > b){a = b;return true;} else return false;}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n;
    string s;
    cin>>n>>s;
    int a=0,c=0,h=0;
    ll cnt=0,d=0,e=0;
    vector<int>X(n),Y(n);
    rep(i,n){
        if(s[i]=='A')a++;
        if(s[i]=='C')cnt+=a;
        if(s[i]=='?'){
            X[i]=a;
            d+=a;
            e++;
        }
    }
    rrep(i,n){
        if(s[i]=='C')c++;
        if(s[i]=='?')Y[i]=c;
    }
    ll ans=cnt+d,f=1;
    rep(i,n){
        if(s[i]=='?'){
            d-=X[i];
            d+=Y[i];
            chmax(ans,cnt+d+(e-f)*f);
            f++;
        }
    }
    cout<<ans<<"\n";
}