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