import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      String s = sc.next();
      int red =0;
      int blue =0;
      for(int i=0;i<s.length();i++){
        if(s.charAt(i)=='0'){
          red++;
        }else{
          blue++;
        }
      }
      System.out.println(Math.min(red,blue)*2);
    }
}