#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

int main() {
    string s; cin >> s;
    int cnt = 0, ans = 0;
    for (char c: s) {
        if (c == 'A') ans += cnt;
        if (c == 'B') cnt++;
    }
    cout << ans << endl;
    return 0;
}