#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=2e5+10;
int n,m;
int g[N],a[N],c[N];
string s;
signed main(){
    cin>>n>>s;
    s='#'+s;
    int res=0;
    for(int i=1;i<=n;i++){
        a[i]=a[i-1],c[i]=c[i-1];
        if(s[i]=='A') a[i]++;
        if(s[i]=='C') c[i]++;
        if(s[i]!='A') res+=a[i];
    }
    int ans=res;
    for(int i=1;i<=n;i++){
        if(s[i]=='?'){
            res-=i-1-c[i-1];
            res+=n-i-(a[n]-a[i]);
            ans=max(ans,res);
        }
    }
    cout<<ans;
    return 0;
}