Check if a Entered Number is Prime Number or Not

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

namespace ConsoleApplication33
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Enter a number to find the factorial: ");
            int a = Convert.ToInt32(Console.ReadLine());
            int dr = 0;
            for (int i = 2; i <= a - 1; i++)
            {
                if (a % i == 0)
                {
                    dr = 1;
                    break;
                }

            }
            if (dr == 1)
            {
                Console.WriteLine("Entered number is not prime.");
            }
            else
            {
                Console.WriteLine("Entered number is prime.");
            }
            Console.ReadKey();
        }

    }
}