static void Main(string[] args)
        {
            Console.WriteLine("Hello C#!");

            Kilogram kg1 = new Kilogram(5);
            Kilogram kg2 = new Kilogram(10);

            Kilogram kg3 = kg1.Add(kg2);
            Kilogram kg4 = kg1 + kg2;

            Console.WriteLine(kg3);
            Console.WriteLine("연산자 오버로딩으로~" + kg4);
        }
 public Kilogram Add(Kilogram target)
 {
     return(new Kilogram(this.mass + target.mass));
 }