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 = 0;
      
      for(int i = 0; i <s.length(); i++){
        if(s.charAt(i) == '1'){
          n += 1;
        } else {
          n -= 1;
        }
      }
      
      System.out.println(s.length() - Math.abs(n));
          
      
    }
}
/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