#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef int64_t lint;
#define rep(i, n) for(int i=0; i<n; i++)
#define repx(i, l, n) for(int i=l; i<n; i++)
#define all(v) v.begin(), v.end()
#define show(x) cout << #x << ": " << x << endl;
#define list(x) cout << #x << ": " << x << "  ";
#define pb push_back
using vi = vector<lint>;
using vvi = vector<vector<lint>>;
template<class T> inline void vin(vector<T>& v) { rep(i, v.size()) cin >> v.at(i); }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<class T> inline void drop(T x) { cout << x << endl; exit(0); }
template<class T> void vout(vector<T> v) { rep(i, v.size()) { cout << v.at(i) << ' '; } cout << endl; }
constexpr lint LINF = LLONG_MAX/2;

int main() {
	lint N;
	string S;
	cin >> N >> S;

	lint a=0, b=0, c=0, x, y, z;
	lint ng = -1, m1, m2, ok = N;
	while (abs(ok - ng) > 2) {
		m1 = (ok + ng*2) / 3;
		m2 = (ok*2 + ng) / 3;
		rep(_, 2) {
			string t;
			rep(i, N) {
				if (S[i] == '?') {
					if (i < m1) t += 'A';
					else t += 'C';
				} else {
					t += S[i];
				}
			}
			vi v(N);
			rep(i, N) {
				if (t[N-1-i] == 'C') v[N-1-i]++;
			}
			rep(i, N-1) v[N-2-i] += v[N-1-i];
			a = 0;
			rep(i, N) {
				if (t[i] == 'A') a += v[i];
			}
			swap(a, b);
			swap(m1, m2);
		}
		if (a < b) ng = m1;
		else ok = m2;
		chmax(c, a);
		chmax(c, b);
	}
	cout << c << endl;
}