#include <iostream>
#include <vector>
#include <string> 
using namespace std;
void setLens(string S,vector<pair<int,int>> &lenVector)
{
	int i;
	int maxLen=0;
	string maxString;
	int aNum=0,bNum=0;
	int len=1;
	for (i=0;i<S.size();i++){
		if (S[i]=='A'){
			aNum++;
		}else{
			bNum++;
		}
		pair<int,int> num;
		num.first=aNum;
		num.second=bNum;
		lenVector.push_back(num);
	}
}
int GetMaxLen(vector<pair<int,int>> &lenVector,char ch){
	int i;
	int MaxLen=0;
	for (i=0;i<lenVector.size();i++){
		if (ch=='A'){
			lenVector[i].first--;
		}else if (ch=='B'){
			lenVector[i].second--;
		}
		if (lenVector[i].first==lenVector[i].second){
			if (MaxLen<2*lenVector[i].first){
				MaxLen=2*lenVector[i].first;
			}
		}
	}
	if (lenVector.size()>0){
		lenVector.erase(lenVector.begin());
	}
	return MaxLen;
}
int main(int argc, char* argv[])
{
	string S;
	cin>>S;
	int len=S.size();
	int aNum=0,bNum=0;
	int i=0;
	for (i=0;i<len;i++){
		if (S[i]=='A'){
			aNum++;
		}else{
			bNum++;
		}
	}
	if (aNum==0 || bNum==0){
		cout<<0<<endl;
		return 0;
	}else if (aNum==bNum){
		cout<<len<<endl;
		return 0;
	}
	vector<pair<int,int>> lenVector;
	setLens(S,lenVector);
	int MaxLen=GetMaxLen(lenVector,' ');		
	for (i=0;i<len;i++){
		int maxLen=GetMaxLen(lenVector,S[i]);
		if (MaxLen<maxLen){
			MaxLen=maxLen;
		}
	}
	cout<<MaxLen<<endl;
	return 0;
}