Double Learn programming C#

using System;
public class exercise55
{
    public static void Main()
    {
        double a, b;
        double area, perimeter, diagonal;

        Console.Write("Enter the width: ");
        a = Convert.ToDouble(Console.ReadLine());

        Console.Write("Enter the height: ");
        b = Convert.ToDouble(Console.ReadLine());

        perimeter = a * 2 + b * 2;
        Console.WriteLine("Perimeter: {0}", perimeter);

        area = a * b;
        Console.WriteLine("Area: {0}", area);

        diagonal = Math.Sqrt(a * a + b * b);
        Console.WriteLine("Diagonal: {0}", diagonal);

    }
}