#include <iostream>
using namespace std;

int f(string S){
  static int ans = 0;
  /*static int two = 0;
  static string tmp = S;
  
  yarinaosi:*/
  
  if(S.length() < 2)return ans;
  
  int tmp = 0;
  for(int i = 0;i < S.length()-tmp;i++){
    if(i != S.length()-1 && S[i] == '0' && S[i + 1] == '1'){
      two = 0;
      ans +=2 ;
      S.erase(S.begin()+i);
      S.erase(S.begin()+i+1);
      tmp += 2;
      i-=2;
    }
    if(i != S.length()-1 && S[i] == '1' && S[i + 1] == '0'){
      two = 0;
      ans +=2 ;
      S.erase(S.begin()+i);
      S.erase(S.begin()+i+1);
      tmp += 2;
      i-=2;
    }
  }
  /*two++;
  if(S == tmp)return ans; else goto yarinaosi;
*/
  return ans;
}


int main(){
  string S;
  cin>>S;    
  cout<<f(S); 
  return 0;
}