import java.util.Scanner;

public class Main {

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();

    int c0 = 0;
    int c1 = 0;

    for (char c : s.toCharArray()) {
      if (c == '0')
        ++c0;
      else
        ++c1;
    }

    System.out.println(2 * Math.min(c0, c1));
  }
}