Help making field amount show negative when number is entered on form.

I am brand new to this. I am using Adobe Acrobat 9, and have converted an excel form into PDF. I am working on a form, and the per diem rate for us is $49 per day. However, if you are provided any of the meals, you have to subtract that from your per diem rate. In one of my rows for the employee to fill in, it asks for the amount total of meals provided. I want the amount in that row for each day to automatically show negative no matter what number is entered. By the way, the only numbers allowed will be 0, 10,15,25,35,40,50. Those are the possible combinations from $0, $10, $15, & $25. But I want it to automatically become negative so that in the Totals Column, it will be subtracted from the total travel expenses. Thank you so much!