#include<bits/stdc++.h>
using namespace std;
string s;
const int u=4e5+7;
int n,ans;
int po[u],a[u],b[u];
int main(){
	cin>>s;
	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++){
		po[i]=n+1;
	}
	for(int i=1;i<=n;++i){
		int xx=a[i]-b[i]+eps;
		po[xx]=min(po[xx],i);
	}
	for(int r=1;r<=n;r++){
		int xx=a[r]-b[r]+eps;
		if(po[xx]<r){
			ans=max(ans,r-po[xx]);
		}	
	}
	cout<<ans;
	return 0;
}