#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
using ll=long long;
using ld=long double;
ld pie=3.141592653589793;
ll mod=998244353;
ll inf=1009999999999999990;
int main(){
    ll n;
    cin >> n;
    string s;
    cin >> s;
    ll a=0;
    ll c=0;
    ll ans=0;
    for (ll i = 0; i < n; i++)
    {
        if (s[i]=='A')
        {
            a++;
        }else{
            ans+=a;
            c++;
        }
    }
    ll aa=0;
    ll now=ans;
    for (ll i = 0; i < n; i++)
    {
        if (s[i]=='?')
        {
            c--;
            now-=aa;
            now+=c;
            ans=max(ans,now);
            aa++;
        }else if (s[i]=='a')
        {
            aa++;
        }else{
            c--;
        }
    }
    cout << ans << endl;
}