import java.util.*;

public class Main {

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

        String s = sc.next();
        char[] chars = s.toCharArray();
        int numOfZero = 0;
        int numOfOne = 0;
        for (int i = 0; i < chars.length; i++) {
            if (chars[i] == '0') {
                numOfZero++;
            }
            if (chars[i] == '1') {
                numOfOne++;
            }
        }
        System.out.println(2 * Math.min(numOfOne, numOfZero));
    }

}
/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