using System;

public class Program

{

    public static void Main()

    {

        do 

        {

            Console.Write("x = ");

                string str = Console.ReadLine();

                if (str == "") break;

            double x = Convert.ToDouble(str);

            Console.Write("The operation is: ");

            string operand = Console.ReadLine();

            while (operand != "+" || operand != "-" || operand != "*" || operand != "/")

            {

                break;

            }

            Console.Write("y = ");

            str = Console.ReadLine();

            double y = Convert.ToDouble(str);

            double result;

            if (operand == "+")

            {

                result = x + y;

                Console.WriteLine(result);

            }

            else if (operand == "-")

            {

                result = x - y;

                Console.WriteLine(result);

            }

            else if (operand == "*")

            {

                result = x * y;

                Console.WriteLine(result);

            }

            if (operand == "/")

            {

                result = x / y;

                Console.WriteLine(result);

            }

​

        }

        while(true);

    }

}