#include<bits/stdc++.h>
using namespace std;

#define rep(i, a, n) for(int i=(a); i<(n); ++i)
#define per(i, a, n) for(int i=(a); i>(n); --i)
#define pb emplace_back
#define mp make_pair
#define clr(a, b) memset(a, b, sizeof(a))
#define all(x) (x).begin(),(x).end()
#define lowbit(x) (x & -x)
#define fi first
#define se second
#define lson o<<1
#define rson o<<1|1
#define gmid l[o]+r[o]>>1
 
using ll = long long;
using ull = unsigned long long;
using pii = pair<int,int>;
using pll = pair<ll, ll>;
using ui = unsigned int;
 
constexpr int mod = 998244353;
constexpr int inf = 0x3f3f3f3f;
constexpr double EPS = 1e-8;
const double PI = acos(-1.0);

constexpr int N = 2e5 + 10;

void _main(){
  int n;
  string s;
  cin >> n >> s;
  ll ans, cur = 0;
  int pre = 0, all = 0, suf = 0;
  rep(i, 0, n){
    if(s[i] == 'A'){
      ++pre;
    } else {
      cur += pre;
      ++suf;
    }
  }
  
  ans = cur;
  pre = 0;
  rep(i, 0, n){
    if(s[i] == 'A'){
      ++pre;
    } else if(s[i] == 'C'){
      --suf;
    } else if(s[i] == '?'){
      --suf;
      cur = cur - pre + suf;
      ++pre;
    }
    ans = max(ans, cur);
  }

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

int main(){
  ios::sync_with_stdio(0);
  cin.tie(0); cout.tie(0);
  _main();
  return 0;
}