import java.util.*;
class Main 
{ 

	
	 
	public static void main(String args[]) 
	{ 
		Scanner s = new Scanner(System.in);
		char[] c = s.next().toCharArray();
		int one=0;
		for(int i=0;i<c.length;i++)
		{
			if(c[i]=='1')
			{
				one++;
			}
			else
			{
				one--;
			}
		}
		System.out.println(c.length-Math.abs(one));

		
		
		
	}
} 

/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