using System;

​

public class PrimeNumberChecker

{

    /// <summary>

    /// Checks if a given positive integer is a prime number.

    /// </summary>

    /// <param name="number">The integer to check.</param>

    /// <returns>True if the number is prime, false otherwise.</returns>

    public static bool IsPrime(int number)

    {

        // Input validation: Check for non-positive integers

        if (number <= 1)

        {

            return false;

        }

​

        // Optimization: Check if the number is 2 or 3

        if (number <= 3)

        {

            return true;

        }

​

        // Optimization: Check if the number is divisible by 2 or 3

        if (number % 2 == 0 || number % 3 == 0)

        {

            return false;

        }

​

        // Optimization: Iterate through odd numbers starting from 5

        // and only up to the square root of the number

        for (int i = 5; i * i <= number; i = i + 6)

        {

            if (number % i == 0 || number % (i + 2) == 0)

            {

                return false;

            }

        }

​

        return true;

    }

​

    public static void Main(string[] args)

    {

        Console.WriteLine("Enter a positive integer: ");

        string input = Console.ReadLine();

​

        if (int.TryParse(input, out int number))

        {

            if (number > 0)

            {

                bool isPrime = IsPrime(number);

​

                if (isPrime)

                {

                    Console.WriteLine("This is a prime number");

                }

                else

                {

                    Console.WriteLine("This is not a prime number");

                }

            }

            else

            {

                Console.WriteLine("Invalid input. Please enter a positive integer.");

            }

        }

        else

        {

            Console.WriteLine("Invalid input. Please enter a valid integer.");

        }

    }

}