using System;

public class Program

{

    public static void Main()

    {

        int num;

        num = int.Parse(Console.ReadLine());

        int x, y;

        if ((num > 999) && (num <10000))

        {

        x = num % 10 + num / 10 % 10;

        y = num / 100 % 10 + num / 1000;

        if (x == y) 

        Console.WriteLine(num % 10 + num / 10 % 10);

        else Console.WriteLine(num);

        }

        else 

        Console.WriteLine("Error");

​

    }

}