prime number program in c #

using System;

class MainClass

{
   public static void Main()

{
int num;
int i;
int factor;
bool isprime;


for(num = 2; num < 20; num++)

{
isprime = true;
factor = 0;


for(i=2; i <= num/2; i++)

            {
if((num % i) == 0)

 {

isprime = false;
factor = i;
}
}

if(isprime)
Console.WriteLine(num + " is prime.");
else
Console.WriteLine("Largest factor of " + num +
" is " + factor);
}

}
}