#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 << endl;

	return 0;
}
//