Primes in Given Range - 100%

using System;
using System.Collections.Generic;


class Primes_in_Given_Range
{
    static void Main(string[] args)
    {
        long firstNum = long.Parse(Console.ReadLine());
        long lastNum = long.Parse(Console.ReadLine());
        List<long> primes = new List<long>();
        for (long num = firstNum; num <= lastNum; num++)
        {
            bool primeOrNot = isPrime(num);
            if (primeOrNot) primes.Add(num);
        }
        Console.WriteLine(string.Join(", ", primes));
    }

    private static bool isPrime(long n)
    {
        int boundary = (int)Math.Floor(Math.Sqrt(n));

        if (n == 0) return false;
        if (n == 1) return false;
        if (n == 2) return true;

        for (int i = 2; i <= boundary; ++i)
        {
            if (n % i == 0) return false;
        }
        return true;
    }
}