I just built this program using Xamarin Studio. It calculates the areas of two different pizzas and tells you which is bigger. It asks you for the shape of the pizza - it only takes "circle" or "square" as shapes, and the side length/radius - called the "length" - of each pizza.

Here's the code:
Code:
using System; // This tells .Net/Mono we want to use the "System" namespace. 

namespace MyFirstCApp // This is a namespace, a place wherein (as you can see) we put classes. 
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			Pizza pizza1 = new Pizza();
			Console.WriteLine ("Pizza 1 shape?");
			pizza1.shape = Console.ReadLine ();
			Console.WriteLine ("Pizza 1 length?");
			pizza1.length = Convert.ToDouble (Console.ReadLine ());
			Pizza pizza2 = new Pizza ();
			Console.WriteLine ("Pizza 2 shape?");
			pizza2.shape = Console.ReadLine ();
			Console.WriteLine ("Pizza 2 length?");
			pizza2.length = Convert.ToDouble (Console.ReadLine ());
			Pizza bestDeal = Pizza.bestDeal (pizza1,pizza2);
			String output;
			if (bestDeal == pizza1) {
				output = "Pizza 1 is the best deal.";
			} else if (bestDeal == pizza2) {
				output = "Pizza 2 is the best deal.";
			} else {
				output = "Both pizzas are the same deal.";
			}
			Console.WriteLine (output);
		}
	}
	class Pizza
	{
		public String shape;
		public double length;
		public double getArea()
		{
			double a = Math.Pow (length, (double)2);
			if (shape == "rectangle") {
				return a;
			} else {
				return a * Math.PI;
			}
		}
		// The "static" keyword allows us to call bestDeal without creating a new Pizza.
		public static Pizza bestDeal(Pizza pizza1,Pizza pizza2)
		{
			Console.WriteLine (pizza1.getArea ());
			Console.WriteLine (pizza2.getArea ());
			if (pizza1.getArea () > pizza2.getArea ()) {
				return pizza1;
			}
			else if (pizza1.getArea () == pizza2.getArea ()) {
				return null;
			} else {
				return pizza2;
			}
		}
	}
}
DISCLAIMER: I kind of stole the idea from my textbook, "Computer Concepts 2014" by Parsons & Oja.