static void Main(string[] args)
        {
            Suma     s1 = new Suma();
            Resta    r1 = new Resta();
            Producto p1 = new Producto();

            s1.Nro1 = 15;
            s1.Nro2 = 10;
            s1.operar();
            s1.imprimir();

            r1.Nro1 = 10;
            r1.Nro2 = 16;
            r1.operar();
            r1.imprimir();

            p1.Nro1 = -10;
            p1.Nro2 = 11;
            p1.operar();
            p1.imprimir();
            Console.ReadKey();
        }