FourDigitNumber

using System;
class FourDigitNumber
{
    static void Main()
    {
        Console.WriteLine("Enter a four-digit number to see some \"magic\" with it.");
        Console.WriteLine("The number musn't start with 0.");
        Console.WriteLine();
        Console.Write("Your number: ");
        string inputNum = Console.ReadLine();
        int number = int.Parse(inputNum);

        if (inputNum.Length > 4)
        {
            Console.WriteLine("The input number is more than 4 digits long! Try again.");
            return;
        }

        int a = number / 1000;
        a %= 10;

        int b = number / 100;
        b %= 10;

        int c = number / 10;
        c %= 10;

        int d = number % 10;

        if (a == 0)
        {
            Console.WriteLine("The number musn't start with 0! Try again.");
            return;
        }

        int sumOfDigits = a + b + c + d;
        Console.WriteLine("The sum of the digits is {0}", sumOfDigits);
        Console.WriteLine("Your number in reversed order is " + d + c + b + a);
        Console.WriteLine("Your number with last digit in front is " + d + a + b + c);
        Console.WriteLine("Your number with second and third digits exchanged is " + a + c + b + d);
    }
}