#include <iostream>
#include <string>
using namespace std;
int main(int argc, char* argv[])
{
	string S;
	cin>>S;
	int i;
	int sLen=S.length();
	int maxLen=0;
	int ANum=0;
	int BNum=0;
	for (i=0;i<sLen;i++){
		if (S[i]=='A'){
			ANum++;
		}else{
			BNum++;
		}
		int MinAB=min(ANum,BNum);
		int MaxAB=max(ANum,BNum);
		if (maxLen<2*MinAB){
			if (MaxAB==MinAB){
				maxLen=2*MinAB;
			}else if ( MaxAB-MinAB==1 &&i<sLen-1){
				maxLen=2*MinAB;
			}
		}
	}
	cout<<maxLen<<endl;
	return 0;
}