#include "bits/stdc++.h"
using namespace std;
#define all(x) begin(x),end(x)
template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << '(' << p.first << ", " << p.second << ')'; }
template<typename T_container, typename T = typename enable_if<!is_same<T_container, string>::value, typename T_container::value_type>::type> ostream& operator<<(ostream &os, const T_container &v) { string sep; for (const T &x : v) os << sep << x, sep = " "; return os; }
#define debug(a) cerr << "(" << #a << ": " << a << ")\n";
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pi;
const int mxN = 1e5+1, oo = 1e9;
int main() {
    int n; cin >> n;
    string s; cin >> s;
    // ac subsequences?
    // ca doesn't make sense.
    // try all options
    // O(n^2) naively
    vector<ll> l(n);
    int a=0;
    for(int i=0;i<n;++i) {
        a+=s[i]=='A' or s[i]=='?';
        l[i]+=(s[i]=='C')?a:0;
        if(i) l[i]+=l[i-1];
    }
    vector<ll> r(n);
    int c=0;
    for(int i=n-1;i>=0;--i) {
        // assume ? replaced by C
        c+=s[i]=='C' or s[i]=='?';
        r[i]+=(s[i]=='A')*c;
        if(i!=n-1) r[i]+=r[i+1];
    }
    ll ans= max(r[n-1],l[0]);
    int ql=0, qr = count(all(s),'?')+count(all(s),'C');
    for(int i=0;i<n-1;++i) {
        ql+=s[i]=='?' or s[i]=='A';
        qr-=s[i]=='?' or s[i]=='C';
        ans = max(ans, l[i]+r[i+1]+ll(ql)*qr);
    }

    cout << ans << '\n';
}