#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <string>
#include <vector>
#include <iostream>
#include <stdlib.h>

using namespace std;

int main()
{
  string s; cin >> s;
  int ans = 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')
          ans ++;
      }
    }
  }

  cout << ans << endl;

  return 0;
}