When a person inputs a number between 3 to 30, another cell shows a number based on what is input.

I am creating a document that, when the user inputs a number between 3 to 30, what ever they choose will determine what will show up in a different cell. This is a character sheet, and these are for the Attributes. I am trying to get the Bonus (Or detriment) to show up in the Adjusted Cell. Thus, if they input 17 into one of the Attribute Cells, +2 shows up in the cell next to it. If they change it later to a smaller or larger number, the Bonus number adjusts accordingly.

Epic Healer