#include<bits/stdc++.h>
using namespace std;
int ans,cnt;
int main(){
	string s;
	cin>>s;
	for(int i=0;i<s.size();i++)
	{
		if(s[i]=='A') ans++;
		if(s[i]=='B') cnt++; 
	 } 
	 int minn=min(ans,cnt);
	 cout<<minn*2;
	return 0;
}
//hgjm