#include <iostream>
#include <string>
using namespace std;

int i,ans,ANS;
string s;

int main() {
	std::cin >> s;
	
	int n = s.size();
	
	for(int j = 0; j < n;j++){
		ANS = 0;
		for(i = 0; i + j < n; i++){
			if(s[i+j] == 'A') ANS++;
			else if(s[i+j] == 'B') ANS--;
			if(ANS == 0) ans = max(ans,i+1);
			
		}
	}
	
	std::cout << ans << std::endl;
}