为什么它向我显示了一个错误，即成本是一个未分配的变量

Console.WriteLine("enter the sirial number of the followin bevrege you want to buy");
Console.WriteLine("beverge                     cost");
Console.WriteLine("1.ness cafe                 2.25");
Console.WriteLine("2.black coffe               2.25");
Console.WriteLine("3.tea                       1.30");
Console.WriteLine("4.hot choclate              2.50");
Console.WriteLine("5.soup                      3.10");
Console.WriteLine("6.coca cola                 3.30");
Console.WriteLine("7.orange juice              3.20");
int sirial;
sirial = int.Parse(Console.ReadLine());
double mouny, cost;
Console.WriteLine("enter how much ouny you enterd to the machine ");
mouny = double.Parse(Console.ReadLine());

    if (sirial == 1 || sirial == 2)
        cost = 2.25;
    if (sirial == 3)
        cost = 1.30;
    if (sirial == 4)
        cost = 2.50;
    if (sirial == 5)
        cost = 3.10;
    if (sirial == 6)
        cost = 3.30;
    if (sirial == 7)
        cost = 3.20;

if (mouny == cost)
    Console.WriteLine(" n/ thank you for buying");