#include <bits/stdc++.h>
using namespace std;
 
int main(){
  string s;
  int i,count;
  cin >>s;
  s = s + "2";
  for(i=0;i<s.size()-2;i++){
    if(s[i] == "2" || s[i+1] == "2")break;
    if(s[i] == "1" && s[i+1] == "0"){
      s.erase(i,2);
      i -= 2;
      count += 2
    }else if(s[i] == "0" && s[i+1] == "1"){
      s.erase(i,2);
      i -= 2;
      count += 2
    }
  }
  cout << count << endl;
}