如何检查输入的金额是否为负数

我需要帮助检查是否输入了负数，但每次我进行更改时都会出现错误。我知道我需要另一个do/while循环，但每次我放入一个amount <= 0，编译器不会读取amount。这是我的工作应用程序的代码:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            double cur1 = 0.85972;
            double cur2 = 1.16316;
            double cur3 = 1.30246;
            double cur4 = 0.76777;
            double cur5 = 0.66010;
            double cur6 = 1.51409;
            double amount;
            double total;
            int input = 1;
            Console.WriteLine("1. Euro to Sterling");
            Console.WriteLine("2. Sterling to Euro");
            Console.WriteLine("3. Euro to Dollar");
            Console.WriteLine("4. Dollar to Euro");
            Console.WriteLine("5. Dollar to Sterling");
            Console.WriteLine("6. Sterling to Dollar");
            Console.WriteLine("7. Exit");

            do
            {
                if (input == 0 || input > 8)
                    Console.WriteLine("You have entered an invalid Option please choose again  " + input + " is not allowed");
                Console.WriteLine("Please Enter your selection 1-6 and 7 to quit:");
                input = int.Parse(Console.ReadLine());

                if (input == 1)
                {
                    Console.WriteLine("Please enter the amount in Euro you want converted into Sterling");
                    amount = double.Parse(Console.ReadLine());
                    total = amount * cur1;
                    Console.WriteLine("The amount of {0} Euros in Sterling at exchange rate {1} is = {2}", amount, cur1, total);
                    Console.WriteLine("Press return to go back to the Menu");
                    Console.ReadKey();
                }
                else if (input == 2)
                {
                    Console.WriteLine("Please enter the amount in Sterling you want converted into Euro");
                    amount = double.Parse(Console.ReadLine());
                    total = amount * cur2;
                    Console.WriteLine("The amount of {0} Sterling in Euros at exchange rate {1} is = {2}", amount, cur2, total);
                    Console.WriteLine("Press return to go back to the Menu");
                    Console.ReadKey();
                }
                else if (input == 3)
                {
                    Console.WriteLine("Please enter the amount in Euro you want converted into Dollar");
                    amount = double.Parse(Console.ReadLine());
                    total = amount * cur3;
                    Console.WriteLine("The amount of {0} Euros in Dollars at exchange rate {1} is = {2}", amount, cur3, total);
                    Console.WriteLine("Press return to go back to the Menu");
                    Console.ReadKey();
                }
                else if (input == 4)
                {
                    Console.WriteLine("Please enter the amount in Dollar you want converted into Euro");
                    amount = double.Parse(Console.ReadLine());
                    total = amount * cur4;
                    Console.WriteLine("The amount of {0} Dollars in Euro at exchange rate {1} is = {2}", amount, cur4, total);
                    Console.WriteLine("Press return to go back to the Menu");
                    Console.ReadKey();
                }
                else if (input == 5)
                {
                    Console.WriteLine("Please enter the amount in Dollar you want converted into Sterling");
                    amount = double.Parse(Console.ReadLine());
                    total = amount * cur5;
                    Console.WriteLine("The amount of {0} Dollars in Sterling at exhange rate  {1} is = {2}", amount, cur5, total);
                    Console.WriteLine("Press return to go back to the Menu");
                    Console.ReadKey();
                }
                else if (input == 6)
                {
                    Console.WriteLine("Please enter the amount in Sterling you want converted into Dollar");
amount = double.Parse(Console.ReadLine());
                    total = amount * cur6;
Console.WriteLine("The amount of {0} Sterling in Dollars at exhange rate {1} is = {2}", amount, cur6, total);
                    Console.WriteLine("Press return to go back to the Menu");
                    Console.ReadKey();
                }
            } while (input != 7);
        }
    }
}

do
{
  Console.WriteLine("Please enter the amount in Euro you want converted into Sterling");
  amount = double.Parse(Console.ReadLine()); 
} while (amount <= 0.0);
total = amount * cur1;
Console.WriteLine("The amount of {0} Euros in Sterling at exchange rate {1} is = {2}", amount, cur1, total);
Console.WriteLine("Press return to go back to the Menu");
Console.ReadKey();