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);
	}
}