import java.util.Scanner;

public class Main {
  public static void main(String args[]) {
    final Scanner scanner = new Scanner(System.in);
    final String input = scanner.next();
    final int count0 = countChar(input, '0');
    final int count1 = countChar(input, '1');
    System.out.println(input.length() - Math.abs(count0 - count1));
  }
  private static int countChar(String str, char target){
    return str.length() - str.replace(String.valueOf(target), "").length();
  }
}
/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