#include <bits/stdc++.h>
#define rep(i,n) for(ll (i)=0;(i)<(n);(i)++)
#define repi(i,a,b) for(ll (i)=(a);(i)<(b);(i)++)
#define SIZE_OF_ARRAY(array)    (sizeof(array)/sizeof(array[0]))
typedef long long ll;
using namespace std;
int main(){
  string s;
  cin >> s;
  int flag = -1;
  int sum = 0;
  int a=0,b=0;
  rep(i,s.size()){
    if(s[i]=='A'){
      a++;
    }else{
      b++;
    }
  }
  
  while(flag<0){
    for(int i=1;i<s.size();i++){
      if(s[i-1]=='B' && s[i]=='A'){
	s[i-1] = 'A';
	s[i] = 'B';
	sum++;
	
      }
      
    }
    
    string str_a = s.substr(0,a);
    string str_b = s.substr(a,b);
    int find_a = str_a.find_last_of('B');
    int find_b = str_b.find_last_of('A');
    if(find_a == std::string::npos){
      if(find_b == std::string::npos){
	flag = 1;
      }
    }
  }
  cout << sum << endl;
    
  return 0;
}