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