#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <cmath>
#include <cassert>
#include <ctime>
#include <utility>
#include <random>
#include <tuple>
#include <bitset>
// #include <atcoder/all>
using namespace std;
// using namespace atcoder;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n) - 1; i >= 0; i--)
#define repk(i, k, n) for (int i = k; i < (int)(n); i++)
#define all(v) v.begin(), v.end()
#define mod1 1000000007
#define mod2 998244353
#define mod3 100000007
#define vi vector<int>
#define vs vector<string>
#define vc vector<char>
#define vl vector<ll>
#define vb vector<bool>
#define vvi vector<vector<int>>
#define vvc vector<vector<char>>
#define vvl vector<vector<ll>>
#define vvb vector<vector<bool>>
#define vvvi vector<vector<vector<int>>>
#define vvvl vector<vector<vector<ll>>>
#define pii pair<int,int>
#define pil pair<int,ll>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define vpii vector<pair<int,int>>
#define vpll vector<pair<ll,ll>>
#define vvpii vector<vector<pair<int,int>>>
#define vvpll vector<vector<pair<ll,ll>>>

template<typename T> void debug(T e){
    cerr << e << endl;
}

template<typename T> void debug(vector<T> &v){
    rep(i, v.size()){
        cerr << v[i] << " ";
    }
    cerr << endl;
}

template<typename T> void debug(vector<vector<T>> &v){
    rep(i, v.size()){
        rep(j, v[i].size()){
            cerr << v[i][j] << " ";
        }
        cerr << endl;
    }
}

template<typename T> void debug(vector<pair<T, T>> &v){
    rep(i,v.size()){
        cerr << v[i].first << " " << v[i].second << endl;
    }
}

template<typename T> void debug(set<T> &st){
    for (auto itr = st.begin(); itr != st.end(); itr++){
        cerr << *itr << " ";
    }
    cerr << endl;
}

template<typename T> void debug(multiset<T> &ms){
    for (auto itr = ms.begin(); itr != ms.end(); itr++){
        cerr << *itr << " ";
    }
    cerr << endl;
}

template<typename T> void debug(map<T, T> &mp){
    for (auto itr = mp.begin(); itr != mp.end(); itr++){
        cerr << itr->first << " " << itr->second << endl;
    }
}

void debug_out(){
    cerr << endl;
}

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T){
    cerr << H << " ";
    debug_out(T...);
}

ll my_pow(ll x, ll n, ll mod){
    //　繰り返し二乗法．x^nをmodで割った余り．
    ll ret;
    if (n == 0){
        ret = 1;
    }
    else if (n % 2 == 1){
        ret = (x * my_pow((x * x) % mod, n / 2, mod)) % mod;
    }
    else{
        ret = my_pow((x * x) % mod, n / 2, mod);
    }
    return ret;
}

ll inv(ll x, ll mod){
    return my_pow(x, mod - 2, mod);
}

int main(){
    ll N;
    cin >> N;
    string S;
    cin >> S;
    vl sum_a(N + 1);
    vl sum_c(N + 1);
    for (ll i = 0; i < N; i++){
        if (S[i] == 'A') sum_a[i + 1] = sum_a[i] + 1;
        else sum_a[i + 1] = sum_a[i];

        if (S[i] == 'C') sum_c[i + 1] = sum_c[i] + 1;
        else sum_c[i + 1] = sum_c[i];
    }
    //debug(sum_a);
    //debug(sum_c);

    ll ans = 0;

    vl a_prd(N, 0);
    vl c_prd(N, 0);
    for (ll i = 0; i < N; i++){
        if (S[i] == '?') a_prd[i] = sum_c[N] - sum_c[i];
        if (S[i] == '?') c_prd[i] = sum_a[i + 1];
    }
    ll hatena = 0;
    for (ll i = 0; i < N; i++){
        if (S[i] == '?') hatena++; 
    }
    //debug(a_prd);
    //debug(c_prd);
    vl a_prd_sum(N, 0);
    vl c_prd_sum(N, 0);
    a_prd_sum[N - 1] = a_prd[N - 1];
    c_prd_sum[0] = c_prd[0];
    for (ll i = 0; i < N - 1; i++){
        c_prd_sum[i + 1] = c_prd_sum[i] + c_prd[i + 1];
    }
    for (ll i = N - 2; i >= 0; i--){
        a_prd_sum[i] = a_prd_sum[i + 1] + a_prd[i];
    }
    //debug(a_prd_sum);
    //debug(c_prd_sum);
    vl hatenas(0);
    rep(i,N){
        if (S[i] == '?') hatenas.push_back(i);
    }
    
    ll ans_base = 0;
    ll a = 0;
    for (ll i = 0; i < N; i++){
        if (S[i] == 'A') a++;
        if (S[i] == 'C') ans_base += a;
    }
    ans = max(ans, ans_base);
    //debug(ans_base);
    
    for (ll i = 0; i <= hatena; i++){
        // どこまでを A で埋めるか
        if (i == 0){
            // すべて C
            ll anssub = 0;
            anssub += c_prd_sum[N - 1];
            ans = max(ans, anssub + ans_base);
        }
        else if (i == hatena){
            // すべて A
            ll anssub = 0;
            anssub += a_prd_sum[0];
            ans = max(ans, anssub += ans_base);
        }
        else{
            ll anssub = 0;
            anssub += (a_prd_sum[0] - a_prd_sum[hatenas[i]]);
            anssub += (c_prd_sum[N - 1] - c_prd_sum[hatenas[i - 1]]);
            anssub += i * (hatena - i);
            ans = max(ans, anssub += ans_base);
        }
    }
    cout << ans << endl;
}