How do you intitialize a variable in a subclass?

The exercise: Challenge Task 1 of 1

The Polygon class represents a 2 dimensional shape. It has a public field named NumSides. Create a new type of polygon called Square. It should have a public readonly field named SideLength that’s initialized using the constructor. Use base to initialize NumSides to 4.

This is my code that i believe was good up to the point before intializing NumSides to 4.: namespace Treehouse.CodeChallenges { class Polygon { public readonly int NumSides;

    public Polygon(int numSides)
    {
        NumSides = numSides;
    }
}
class Square : Polygon
{
    public Square(int numSides) : base (numSides)
    {

    }
    public readonly int SideLength;    
    public Square (int sideLength)
    {
      SideLength = sideLength;  
    }
}

}

My Problem: I am having trouble figuring out how to intialize numSides to 4 per the instructions. In the lesson exercise we used seperate files. MapLocation.cs was a subclass of Point.cs. Both of these files were called out in Game.cs. I have tried a host of different things including: setting numSides = 4; inside of public Square putting Polygon numSides = new Polygon(4); outside of class Polygon

It made much more sense to me when separate files were used. Any thoughts/insights would be greatly appreciated.