#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef pair<ll,ll> P;
typedef vector<ll> VI;
typedef vector<VI> VVI;
#define REP(i,n) for(int i=0;i<(n);i++)
#define ALL(v) v.begin(),v.end()
template<typename T> bool chmax(T& x, const T& y){return (x<y)?(x=y,true):false;};
template<typename T> bool chmin(T& x, const T& y){return (x>y)?(x=y,true):false;};
constexpr ll MOD=998244353;
constexpr ll INF=2e18;

int main(){
    int n; cin >> n;
    string s; cin >> s;
    VI a(n+1,0), c(n+1,0);
    REP(i,n) a[i+1]=a[i]+(s[i]!='C'?1:0);
    REP(i,n) c[n-i-1]=c[n-i]+(s[n-i-1]!='A'?1:0);
    ll x=0;
    REP(i,n){
        if(s[i]=='A')
            x+=c[i];
    }
    ll ans=x;
    REP(i,n){
        if(s[i]=='?'){
            x+=c[i+1];
            x-=a[i];
        }
        chmax(ans,x);
    }
    cout << ans << endl;
    return 0;
}