#include<cstdio>
#include<cstring>
#include<iostream>
#include<cmath>
#include<ctime>
#include<string>
#include<algorithm>
#include<vector>
#include<queue>
#include<stack>
#include<map>
using namespace std;
typedef long long LL;
const int MR=2e5+10;
int n;
string s;
int a[MR],c[MR];
int main(){
    cin>>n>>s;
    LL sum=0;
    for(int i=1;i<=n;i++){
        a[i]=a[i-1];
        if(s[i-1]=='A') a[i]++;
        c[i]=c[i-1];
        if(s[i-1]=='C'){
            c[i]++;
        }
        if(s[i-1]!='A') sum+=a[i-1];
    }
    LL ans=sum;
    //printf("i=%d sum=%lld\n",0,sum);
    for(int i=1;i<=n;i++){
        if(s[i-1]=='?'){
            sum-=i-1-c[i-1];
            sum+=n-i-(a[n]-a[i]);
        }
        //printf("i=%d sum=%lld\n",i,sum);
        ans=max(ans,sum);
    }
    cout<<ans<<endl;
    return 0;
}