import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		char[] s = br.readLine().toCharArray();
		br.close();

		int[] a = new int[n + 1];
		for (int i = 0; i < n; i++) {
			a[i + 1] = a[i];
			if (s[i] == 'A') {
				a[i + 1]++;
			}
		}
		int[] c = new int[n + 1];
		for (int i = n - 1; i >= 0; i--) {
			c[i] = c[i + 1];
			if (s[i] == 'C') {
				c[i]++;
			}
		}

		long val = 0;
		int cnt = 0;
		int cq = 0;
		for (int i = 0; i < n; i++) {
			if (s[i] == 'A') {
				cnt++;
			} else {
				val += cnt;
				cq++;
			}
		}

		long ans = val;
		cnt = 0;
		for (int i = 0; i < n; i++) {
			if (s[i] == 'A') {
				cnt++;
			} else if (s[i] == '?') {
				val -= cnt;
				cq--;
				val += cq;
				cnt++;
				ans = Math.max(ans, val);
			} else {
				cq--;
			}
		}
		System.out.println(ans);
	}
}