import java.util.*;


public class Main {
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        String a = sc.next();
        int beforeSize = a.length();
        int zero = 0;
        int one = 0;
        for (char c : a.toCharArray()) {
            if (c == '0') {
                zero++;
            } else {
                one++;
            }
        }
        int sub = zero - one;
        if (sub < 0) {
            sub = sub * -1;
        }
        System.out.println(beforeSize - sub);
    }
}

/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