#include <iostream>
#include <string>
using namespace std;

int main(void){
    string s;
    cin >> s;
    int cnt=0;
    for(int i=1;i<s.length();i++){
        int idx = i;
        if(s[idx] == 'A'){
            while(s[idx-1] == 'B'){
                char c = s[idx];
                s[idx] = s[idx-1];
                s[idx-1] = c;
                idx--;
                cnt++;
            }
        }
    }
    cout << cnt << endl;
    return 0;
}