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

template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
signed main(){
  string s;
  cin>>s;
  Int n=s.size();
  Int ans=0;
  map<Int, Int> m;
  m[0]=-1;
  Int k=0;
  for(Int i=0;i<n;i++){
    if(s[i]=='A') k++;
    else k--;
    if(!m.count(k)) m[k]=i;
    chmax(ans,i-m[k]);
  }
  cout<<ans<<endl;
  return 0;
}