import java.util.*;

public class Main {

  public static void main (String[] args) {

    Scanner sc = new Scanner(System.in);
    
    String S = sc.next();

    sc.close();

    String[] s = S.split("");

    int red = 0;
    int blue = 0;

    for (int i = 0; i < s.length; i++) {
      if(s[i].equals("0")){
        red++;
      } else {
        blue++;
      }
    }

    System.out.println(s.length - Math.abs(red - blue));

  }

}