#include <iostream>
#include <bits/stdc++.h>
using namespace std;

int c,n,ans,p[210000];

int main(){
	string s;
	cin >> s;
	
	n = s.size();
	for(int i = 0; i < n; i++){	
		if(s[i] == 'A'){
			c++;
			p[i] = c;
		}
		else if(s[i] == 'B'){
			c--;
			p[i] = c;
		}
	}
	for(int i = 0; i < n; i++){
		for(int j = 0; i+j < n; j++){
			if(p[i] == p[i+j]) ans = max(ans,j);
		}
	}
	cout << ans << endl;
}