#include<bits/stdc++.h>
using namespace std;
using Int = long long;
//INSERT ABOVE HERE
signed main(){
  string s;
  cin>>s;
  Int cnt=0;
  while(1){
    Int flg=0;
    for(Int i=1;i<(Int)s.size();i++){
      if(s[i-1]=='B'&&s[i]=='A'){
	flg=1;
	cnt++;
	swap(s[i-1],s[i]);
      }
    }
    if(!flg) break;
  }
  cout<<cnt<<endl;
  return 0;
}