using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Enter the first num");

        int num1 = int.Parse(Console.ReadLine());

        Console.WriteLine("Enter the second num");

        int num2 = int.Parse(Console.ReadLine());

        Console.WriteLine("Enter the second num");

        int num3 = int.Parse(Console.ReadLine());

        int diff1 = num2 - num1;

        int diff2 = num3 - num2;

        if (diff1 = diff2)

        {

            int num4 = num3 + diff1;

            Console.WriteLine("Number 4 is = {0}" , num4);

        }

            else if ( diff1 > diff2)

            {

                Console.WriteLine("Error");

            }

    }

}