Use one field to determine which calculation a different field is used in?

So i'm making a PDF for a game i'm making. I make a drop down field that decides whether a gear used is armor or shield. If it's armor, then the Defense field I added will be added to the armor spot, and if its a shield, to the shield spot. How would I do that?

Basically, if something is an armor, and it gives two defense, it shows +2 in the armor section. If its a shield and gives +2 armor, it shows + 2 in the shield section. I'm new to this program, and don't know any javascript.