Calculated field adds a negative number instead of subtracting it
I have a purchase order form that I created, with calculated fields to take the quantity times the price to get an extended amount. I then have a calculated subtotal field at the bottom that sums all of the extended quantities. For the subtotal field, I selected the "Value is the sum of the following fields".

However, if there is a credit line and I enter a negative number, it adds the amount instead of subtracts it. If I take the minus sign out, then it subtracts it.
I'm sure I'm missing something really simple here, but I'd appreciate it if someone could help me out. Do I need to do an If/Then statement? What would that look like?
Thanks.
Lisa
