Validation Question

Validation Question

I have a form Expenses. I have two field I need to work with, (1) Reason for expense and (2) order#.
I need to write a script that when you select a reason " Fees", that you MUST enter an order # before yo can submit.
my script said;

(Order is a number field)
(Reason is a drop down select-Boolean)

order_Temp =input.order;
if reason = "Fee"
{
      order_Temp=input.order>0;
      alert "an order must be inputted when "Fees are selected"
}

I keep getting errors
any idea what I am doing incorrectly?