#include <bits/stdc++.h>
#define  rep(i,n) for(li i=0;i<(n);i++)
#define  red(i,n) for(li i=(n)-1;i>=0;i--)
#define  all(a)   begin(a),end(a)
#define  pb(a)    push_back(a) 
#define  fi first
#define  se second 
using namespace std;
using li = long long;
using ul = unsigned long long;
using vi = vector<li>;
using vs = vector<string>;
inline ul gcd(ul a,ul b) { return b?gcd(b,a%b):a; }
const int INF=1000000009;


int main()
{
    int ans=0;
    string s;
    cin>>s;
    for(bool F=1;F;){
        F=0;
        for(int i=0;i<(int)s.size()-1;i++) if(s[i]=='B' && s[i+1]=='A')swap(s[i],s[i+1]),ans++,F=1;
    }
    cout<<ans<<endl;
    return 0;
}