#include<bits/stdc++.h>
using namespace std;
const int N=4e5+7;
string s;
int pos[N],a[N],b[N];
int main(){
	cin>>s;	
	int n=s.size();
	s=" "+s;
	for(int i=1;i<=n;i++){
		a[i]=a[i-1]+(s[i]=='A');
		b[i]=b[i-1]+(s[i]=='B');		
	}
	int eps=200000;
	for(int i=0;i<=400000;i++){
		pos[i]=n+1;
	}
	for(int i=1;i<=n;i++){
		int x=a[i]-b[i]+eps;
		if(pos[x]==n+1)pos[x]=i;
	}
	int ans=0;
	for(int r=1;r<=n;++r){
		int x=a[r]-b[r]+eps;
		if(pos[x]<r){
			ans=max(ans,r-pos[x]);
		}
	}
	cout<<ans;
	return 0;
}