#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define ALL(v) v.begin(),v.end()
typedef long long ll;

#include<bits/stdc++.h>
using namespace std;

int main(){
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);

  string s;
  cin>>s;
  int n=s.size();
  
  vector<int> A(n+1);
  rep(i,n){
    if(s[i]=='A') A[i+1]=A[i]+1;
    else A[i+1]=A[i]-1;
  }
  
  map<int,int> m1,m2;
  m1[0]=-1,m2[0]=-1;
  int ans=0;
  for(int i=0;i<n;i+=2){
    if(m1.count(A[i+1])) ans=max(ans,i-m1[A[i+1]]);
    else m1[A[i+1]]=i;
  }
  for(int i=1;i<n;i+=2){
    if(m2.count(A[i+1])) ans=max(ans,i-m2[A[i+1]]);
    else m2[A[i+1]]=i;
  }
  cout<<ans<<endl;
      
  return 0;
}