using System;

// my own implementation of is prime number

// you can check it on http://www.math.com/students/calculators/source/prime-number.htm

​

public class Program

{

    public static bool IsPrimeNumber(int n)

    {

        if (n < 2) return false; // should be >= 2

        if (n % 2 == 0) return false; // should not be divisible by 2

        for (int i = 3; i < n / 2; i += 2) // should not be divisible by any odd numbers

        {

            if (n % i == 0) return false;

        }

        return true;

    }

    public static void Main()

    {

        int n = 5;

        Console.WriteLine($"input: {n}, {(IsPrimeNumber(n) ? "is" : "not")} a prime number");

​

        n = 117;

        Console.WriteLine($"input: {n}, {(IsPrimeNumber(n) ? "is" : "not")} a prime number");

    }

}