import java.util.*;

class Main{
	public static void main(String args[]) {
		Scanner sc=new Scanner(System.in);
		String s=sc.next();
		int count=0;
		for(int i=0;i<s.length();++i) {
			if (s.charAt(i)=='1')++count;
		}
		System.out.println(Math.min(s.length()-count, count)*2);

	}
	
}
