#include<iostream>
#include<vector>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<string>
#include<sstream>
#include<cmath>
#include<numeric>
#include<map>
#include<stack>
#include<queue>
using namespace std;
int inf = 1000000000;

int main(void) {
  string s;
  cin >> s;
  bool f = true;
  int cnt = 0;
  while( f ){
    f = false;
    for(int i=0; i<s.size()-1; i++){
      if( s[i] == 'B' && s[i+1] == 'A' ){
        swap(s[i], s[i+1]);
        cnt++;
        f = true;
      }
    }
  }
  cout << cnt << endl;

  return 0;
}

// EOF