Simple Formula Not Working (Error Message)...

Hi,

I am trying to create a very simple formula but am getting an error message each time.
Basically I have 2 drop down fields called Hours_Worked and Basic_Pay and I am trying use a formula to calculate the 'Amount Due'. So...as far as I can tell it should be (Hours_Worked * Basic_Pay).
Eg: If Hours_Worked = 10 and Basic_Pay = 2 the formula should be:
(Hours_Worked * Basic_Pay) - (10 * 2) - So the Amount Due should = 20.

However, every time I try this formula I get the following error message:

"Error in statement at line number 0
The template variable Hours_Worked is of type STRING. But a Integer Expression was expected"

Can anyone tell me where Im going wrong?