#pragma GCC target ("avx2")
#pragma GCC optimize ("O3")
#pragma GCC optimize("Ofast")
#pragma GCC optimize ("unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
using namespace std;
#define io ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL) 
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define endl '\n'
typedef long long ll;
#define mod1 (ll)1000000007
#define mod2 (ll)998244353
#define pll pair<ll,ll>
typedef long double lb;
typedef tree<
pair<ll, ll>,
null_type,
less<pair<ll, ll>>,
rb_tree_tag,
tree_order_statistics_node_update> ordered_set;
#define eps (lb)(1e-9)
struct custom_hash {
    static uint64_t splitmix64(uint64_t x) {
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }

    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};
// Operator overloads
template<typename T1, typename T2> // cin >> pair<T1, T2>
istream& operator>>(istream &istream, pair<T1, T2> &p) { return (istream >> p.first >> p.second); }

template<typename T> // cin >> vector<T>
istream& operator>>(istream &istream, vector<T> &v)
{
    for (auto &it : v)
        cin >> it;
    return istream;
}

template<typename T1, typename T2> // cout << pair<T1, T2>
ostream& operator<<(ostream &ostream, const pair<T1, T2> &p) { return (ostream << p.first << " " << p.second); }
template<typename T> // cout << vector<T>
ostream& operator<<(ostream &ostream, const vector<T> &c) { for (auto &it : c) cout << it << " "; return ostream; }

// Utility functions
template <typename T>
void print(T &&t)  { cout << t << "\n"; }
template <typename T, typename... Args>
void print(T &&t, Args &&... args)
{
    cout << t << " ";
    print(forward<Args>(args)...);
}
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
ll random(ll p){ // gives random number in [0,p]

 return uniform_int_distribution<ll>(0, p)(rng);
}
ll dp[1<<18];
ll calc[1<<18];
ll n; string s;
void solve();
int main() {
io;
ll t=1,n=1;
// cin>>t;
while (t--){
     solve();
    }
     return 0;
}
void solve(){
    cin>>n>>s;
    // dp[i] -> total AC in s[0..i] if all ? are A
    // calc[i] ->  total AC in s[i...] if all ? are C
    ll ans=0;
    ll cnt=0;
    for(ll i(0);i<n;++i){
        if(i==0){
            dp[i]=0;
        }
        else if(s[i]!='C'){
            dp[i]=dp[i-1];
        }
        else{
            dp[i]=cnt+dp[i-1];
        }
        cnt+=(s[i]!='C');
        ans=max(ans,dp[i]);
    }
    cnt=0;
    for(ll i(n-1);i>-1;--i){
        if(i==n-1){
            calc[i]=0;
        }
        else if(s[i]!='A'){
            calc[i]=calc[i+1];
        }
        else{
            calc[i]=cnt+calc[i+1];
        }
        cnt+=(s[i]!='A');
        ans=max(ans,calc[i]);
    }
    vector<ll>va(n),vc(n),p(n);
    for(ll i(0);i<n;++i){
        if(i==0){
            va[i]=(s[i]=='A');
            vc[i]=(s[i]=='C');
            p[i]=(s[i]=='?');
        }
        else{
            va[i]=(s[i]=='A')+va[i-1];
            vc[i]=(s[i]=='C')+vc[i-1];
            p[i]=(s[i]=='?')+p[i-1];
        }
    }
    for(ll i(0);i<(n-1);++i){
        ans=max(ans,dp[i]+calc[i+1]+(va[i]+p[i])*(vc.back()+p.back()-vc[i]-p[i]));
    }
    cout<<ans<<endl;




}
// Do not get stuck on a single approach for long, think of multiple ways