#include <bits/stdc++.h>
#include <atcoder/all>

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

using namespace std;
using namespace atcoder;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repll(i, n) for (long long i = 0; i < (long long)(n); i++)
#define rep2(i, n, m) for (int i = n; i < (int)(m); i++)
#define repll2(i, n, m) for (long long i = n; i < (long long)(m); i++)
#define all(v) v.begin(),v.end()
using ll=long long;
using ld=long double;
using vi=vector<int>;
using vvi=vector<vi>;
using vvvi=vector<vvi>;
using vl=vector<ll>;
using vvl=vector<vl>;
using vvvl=vector<vvl>;
using vld=vector<ld>;
using vvld=vector<vld>;

int dx[8]={1,0,-1,0,1,1,-1,-1};
int dy[8]={0,1,0,-1,1,-1,1,-1};

const double PI = acos(-1);
//const ll MOD=1e9+7;
//const ll MOD=998244353;
const ll INF=(1LL<<60);
const int INF2=(1<<30);
//using mint=modint1000000007;
//using mint=modint998244353;

int n;
string s;
ll cnt_a[200010],cnt_c[200010],cnt_q[200010];



int main() {
    ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    cin>>n;
    cin>>s;
    cnt_a[0]=0,cnt_c[0]=0,cnt_q[0]=0;
    rep(i,n){
        cnt_a[i+1]=cnt_a[i];
        cnt_c[i+1]=cnt_c[i];
        cnt_q[i+1]=cnt_q[i];
        if(s[i]=='A')cnt_a[i+1]++;
        else if(s[i]=='C')cnt_c[i+1]++;
        else cnt_q[i+1]++;
    }
    ll ans=-INF;
    ll ca=cnt_a[n],cc=cnt_c[n],cq=cnt_q[n];
    ll base=0;
    for(int i=1;i<n+1;i++){
        if(s[i-1]=='A')base+=cc-cnt_c[i]+cq-cnt_q[i];
        //else base+=cnt_a[i];
    }
    //cout<<base<<endl;
    ans=base;
    for(int i=1;i<n+1;i++){
        //i+1より左の?は全てA,右はC
        if(s[i-1]!='?')continue;
        base-=cnt_a[i]+cnt_q[i-1];
        base+=cc-cnt_c[i]+cq-cnt_q[i];
        ans=max(ans,base);
        //cout<<i<<" "<<base<<endl;
    }
    cout<<ans<<endl;
    return 0;
}