using System;

​

public class Program

{

    public static void Main()

    {

        for(var i = 0; i <= 25; i++)

        {

            var number = i;

            var closestPrime = GetClosestPrime(number);

            if(number == closestPrime)

            {

                Console.WriteLine("The number " + number + " IS a Prime number!");

            }

            else

            {

                Console.WriteLine("The closest Prime number to " + number + " is: " + closestPrime);

            }

        }

    }

    public static int GetClosestPrime(int number)

    {

        var closestPrime = 0;

        if(isPrime(number))

        {

            closestPrime = number;

        }

        else

        {

            var i = number + 1;

            var j = number - 1;

            while (closestPrime == 0)

            {

                if(isPrime(i))

                {

                    closestPrime = i;

                    break;

                }

                if(isPrime(j))

                {

                    closestPrime = j;

                    break;

                }

                i++;

                j--;

            }

        }

        return closestPrime;

    }

    private static bool isPrime(int number)

    {

        if (number < 2)

        {

            return false;

        }

        if (number % 2 == 0)

        {

            return false;

        }

        for(var i = 3; i <= Math.Sqrt(number); i+=2)

        {

            if (number % i == 0)

            {

                return false;

            }

        }

        return true;

    }

}