FourDigitNumber

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

/*
 * Write a program that takes as input a four-digit number in format abcd (e.g. 2011) and performs the following:
 * •  Calculates the sum of the digits (in our example 2+0+1+1 = 4).
 * •  Prints on the console the number in reversed order: dcba (in our example 1102).
 * •  Puts the last digit in the first position: dabc (in our example 1201).
 * •  Exchanges the second and the third digits: acbd (in our example 2101).
 * The number has always exactly 4 digits and cannot start with 0.
*/

class FourDigitNumber
{
    static void Main()
    {
        int number;
        int reverseNum = 0;
        int sumNum = 0;
        string separator = new string('-',10);

        Console.WriteLine("Provide a 4 digit positive integer:");
        while (!int.TryParse(Console.ReadLine(), out number))
        {
            Console.WriteLine("Please, enter valid integer.");
        }

        //sum
        int sumNumber = number;
        while (sumNumber > 0)
        {
            sumNum += sumNumber % 10;
            sumNumber = sumNumber / 10;
        }
        Console.WriteLine("\n sum: {0}\n{1}", sumNum, separator);

        //reverse
        int revNumber = number;
        while (revNumber > 0)
        {
            reverseNum = (reverseNum * 10) + (revNumber % 10);
            revNumber = revNumber / 10;
        }
        Console.WriteLine("\n reversed: {0}\n{1}", reverseNum, separator);

        //last to first
        string numToStr = number.ToString();
        Console.WriteLine("\n last to first: {0}\n{1}", numToStr.Last() + numToStr.Substring(1, numToStr.Length - 2) + numToStr.First(), separator);

        //excange 2nd and 3rd
        Console.WriteLine("\n excange 2nd and 3rd: {0}\n{1}", numToStr.First() + numToStr.Substring(2, 1) + numToStr.Substring(1, 1) + numToStr.Last(), separator);

        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}