Request for help with custom adobe acrobat script for totting up holiday cost receipt

I need to use a custom adobe acrobat script to assist with totting up total holiday costs for a receipt. I have the initial deposit which is then added to the holiday cost giving the Total amount. The holiday cost could either be paid in one amount or up to three installmants. so sub total D is sum of the holiday cost OR sum of Inst A + Inst B + Inst C. With the total being the deposit plus the sub total. Any help would be greatly appreciated. Many thanks.