#include<bits/stdc++.h>
using namespace std;
const int N=4e5+7;
int pos[N],a[N],b[N];
string s;
int main()
{
    ios::sync_with_stdio(false); 
    cin.tie(0);
    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 eps=200000,ans=0;
	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;
	}
	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;
}