import java.util.Scanner;
public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   String S = sc.next();
   int N = S.length();

   int count =0;
   for(int i =0;i<N;i++)
   {
    if(S.charAt(i)=='0') count++;
   }
   if(N-2*count>0) System.out.println(2*count);
   else System.out.println(2*(N-count));
 }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz