#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n; cin >> n;
	string s; cin >> s;
	vector<long long>ca(n + 1), cc(n + 1), cx(n + 1);
	long long tmp = 0;
	vector<int>pos;
	rep(i, n) {
		ca[i + 1] = ca[i];
		cc[i + 1] = cc[i];
		cx[i + 1] = cx[i];
		if ('A' == s[i])ca[i + 1] += 1;
		else if ('C' == s[i])cc[i + 1] += 1;
		else pos.push_back(i);
		if ('C' == s[i])tmp += ca[i + 1] - ca[0];
	}
	long long ans = tmp;
	rep(i, pos.size() + 1) {
		long long sub = tmp;
		int j = pos.size() - i;
		sub += (long long)i * j;
		if (0 != i)sub += (long long)i * (cc[n] - cc[pos[i - 1]]);
		if (pos.size() != i)sub += (ca[pos[i]] - ca[0]) * j;
		chmax(ans, sub);
	}
	cout << ans << endl;
	return 0;
}