IF Function or Something

[babyjay]

I need help in creating a formula. I need a formula(s) where we input inventory on hand (cloumn B&C) then it compares to par levels we have identified to have on hand (columns D &E) then determines the amount needed to purchase (column F). In identifying par levels we do so by case and each. Listed below is the fist few columns of the spreadsheet. Corn Flakes case comes with 4 each, Baby Corn case is 6 each #10 cans, and Croutons case comes with 8 each 2# bags.


___A________B___C____D____E_____F
Item Name__On Hand_____Par_____ Order
_________ Case_Each_Case_Each_______

Corn Flakes----0------1------1------0--------------
Baby Corn-------------4------1------0--------------
Croutons-------1------2------2------o--------------

I cannot figure this one out.