import java.util.Arrays;
import java.util.LinkedList;
import java.util.Scanner;

public class Main {
	
	public static final long MOD = 1000000007;
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int A = 0, B = 0;
		for(final char c : sc.next().toCharArray()){
			if(c == '0'){ A++; }
		}
		for(final char c : "1000000007".toCharArray()){
			if(c == '0'){ B++; }
		}
		
		System.out.println(Math.abs(A - B));
	}
}