import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        String s = sc.next();
        int zero = 0;
        int one = 0;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '0') {
                zero++;
            } else {
                one++;
            }
        }
        System.out.println(Math.min(zero, one) * 2);

       
    }
}