import java.util.Scanner;

/**
 * Unification
 *
 */
public class Main {
  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    String[] target = scanner.nextLine().split("");
    int numOfZero = 0;
    int numOfOne = 0;
    
    for (String s : target) {
      if ("0".equals(s)) numOfZero++;
      if ("1".equals(s)) numOfOne++;
    }
    
    
    if (numOfOne >= numOfZero) {
      System.out.println(2 * numOfZero);
    } else {
      System.out.println(2 * numOfOne);
    }
  }
}
/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