How do I calculate fields

I have a form created to make a list of food items in a fridge and I want the list to automatically calculate how much I need to order

example

I always need 10 Chickens, if someone fills out that we have 4 chickens if the Chicken box it will automatically say we need 6 chickens 

how do I do this I was trying in conditions but could not figure it out PLEASE HELP

Here's how to perform calculation in your form.

http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form

Example: This is base from your example: