Acrobat 9.5 - modifying text values based on input from another text box

[Roguish1]

I have a simple form with only a few text boxes, what I need is when a user enters a numerical value in one text box another text box is populated with a non calculated value.

textbox1 user enters a value between 1 and 18

textbox2 will then display a value based on what data was entered in textbox1.
1–3 = -3
4–5 = -2
6–8 = -1
9–12 = 0
13–15 = +1
16–17 = +2
18 = +3
this data is not based on a calculation but will be used in a later calculation.
I am about use a get.field but I am stuck on the if or if/else to displace the proper value.

Any help would be great appreciated,
Thank you