C# program problem!

Question

Hi to everyone,

I'm newbie here and in C# also. I'd like to give you congratulations about this forum!
I'd like also to ask you how to insert values in an Array so i can create a dynamic array...
My problem is to make a program to give my grades and return the sum of them!
Source code is provided:
Any help please???
Thanx in advance...

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

namespace Courses
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Give me the number of courses you have passed:");
            String a = Console.ReadLine();
            int b = Convert.ToInt16(a);
            int sum = 0;
            if ((b <= 0) || (b > 20))
            {
                Console.WriteLine("You gave me wrong number of courses,try again!!!");
            }
            else
            {
               
                int i = 1;
                while (i <= b)
                {
                    Console.WriteLine("Give me the grades of your courses,pressing Enter: " + i);
                    i++;
                }
               
                int[] num = new int[20];
                for (int j = 1; j < i; j++)
                {
                    string source = Console.ReadLine();
                   
                    int pin = num[j];
                    sum = sum + num[j];

                }
                    Console.WriteLine("The suma is: " + sum);
               
                    Console.ReadKey();

            }
        }
    }
}