Ex!1รอบ2

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication7
{
    class Program
    {
        static void Main(string[] args)
        {
            int x=-1;
            Console.Write("\nInput Value : ");
            int a = int.Parse(Console.ReadLine());
            do{
                Console.Write("Input Opertor : ");
                string op = Console.ReadLine();
                calculate(op, ref a,ref x);
              } while (x != 0);
        Console.Write("End Value is {0}",a);
        Console.ReadLine();
        }
        static int calculate(string op, ref int a,ref int x)
        {
           int b;
           switch (op)
            {
                case "*": Console.Write("Input Number : ");
                    b = int.Parse(Console.ReadLine());
                    a = a * b;
                    Console.WriteLine("Present Value = {0} ", a);
                    break;
                case "/": Console.Write("Input Number : ");
                    b = int.Parse(Console.ReadLine());
                    a = a / b;
                    Console.WriteLine("Present Value = {0} ", a);
                    break;
                case "-": Console.Write("Input Number : ");
                    b = int.Parse(Console.ReadLine());
                    a = a - b;
                    Console.WriteLine("Present Value = {0} ", a);
                    break;
                case "+": Console.Write("Input Number : ");
                    b = int.Parse(Console.ReadLine());
                    a = a + b;
                    Console.WriteLine("Present Value = {0} ", a);
                    break;
                default: x = 0; break;
                }
            return a;

        }
    }
}