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

int main(){
    string s;
    cin >> s;

    int fp = 0,
        bp = s.length() - 1,
        res = 0;

    for(int i=0; i<s.length(); i++){
        if(s[i] == 'B')
            for(int j=i+1; j<s.length(); j++){
                if(s[j] == 'A'){
                    swap(s[i], s[j]);
                    res += j-i;
                    break;
                }
            }
    }

    cout << res << endl;
}