#include <iostream>
#include <string>

void solve() {
    std::string s;
    std::cin >> s;
    int n = s.length();

    int ans = 0;
    for (int j = 0; j < n; ++j) {
        for (int i = 0; i < j; ++i) {
            if (s[i] == 'B' && s[j] == 'A') ++ans;
        }
    }

    std::cout << ans << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}