import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		char[] s1 = sc.next().toCharArray();
		int ans = 0;
		int c1 = 0;
		int c0 = 0;
		for(int i = 0; i < s1.length; i++) {
			if(s1[i] == '1') {
				if(c0 > 0) {
					c0--;
					ans++;
				}else {
					c1++;
				}
			}else {
				if(c1 > 0) {
					c1--;
					ans++;
				}else {
					c0++;
				}
			}
		}
		
		System.out.println(ans * 2);
	}
}
/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