Help with Adobe Professional 7 form calculation

I know how to add sums by going into the 'Calculate' tab of the 'Text Field Properties' to auto-populate fields on a form in Adobe Acrobat Professional 7 (summing up fields by adding them together), but is there a way to subtract sums as well? I don't see this in the drop down menu. I know how to do it in LiveCycle, but was wondering if it is possible in Acrobat Professional?