#include <iostream>
#include <string>
#include <algorithm>

int main() {
    std::string S;
    std::cin >> S; // Read the length (unused)
    std::cin >> S; // Read the actual string

    long long A = 0, C = 0, a = 0, n = 0;
    char L = 'A';
    for (char s : S) {
        if (s == L) A++;
        else {
            C++;
            n += A;
        }
    }

    long long c = C, m = n;
    for (char s : S) {
        if (s < L) n += c - 1 - a;
        c -= s > L;
        a += s == L;
        m = std::max(n, m);
    }

    std::cout << m << std::endl;
    return 0;
}