
import java.util.Scanner;

public class Main{
	
 public static void main(String[] args) {
	 Scanner sc=new Scanner(System.in);
	 String S=sc.next();
	 int A=0;
	 int B=0;
	 char[] c=S.toCharArray();
	 for(int i=0;i<S.length();i++) {
		 if(c[i]=='1') {
			 A++;
		 }else {
			 B++;
		 }
	 }
	 
	 System.out.println(2*Math.min(A,B));
 }
	


}