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