import java.util.*;

public class Main
{
	public static void main (String[] args)
	{
		Scanner sc = new Scanner (System.in);
		String str = sc.nextLine();
		int n = str.length();
		int red = 0;
		int blue = 0;

		//0=red	1=blue

			for (int i = 0; i < n; i++)
			{
				if (str.substring (i,i+1).equals ("0"))
					red++;
				else
					blue++;
			}

		int ans = 0;
		if (red>blue)
			ans = n-(red-blue);
		else
			ans = n-(blue-red);
		System.out.println (ans);
	}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz