Need help with formula

I am trying to create a formula for one of my fields so that it automatically calculates it from 2 other fields. I do not know java script and am just trying to put it in as an algebra formula but of course it's not working. This is what I have:

I'm trying to get the Y column to populate off of what is entered in the X column. The field is called X1 and the number I'm subtracting is the 36 which is a field called ARC and of course the result is call Y1.

This is what the formula looks like in the Y field now under the calculate tab:  X1-ARC=RESULT/2, so of course you can see the result is 16 but needs to be 8.

Can anybody help with this please? Thanks. Sorry but I'm not a Java guru, just an Engineer trying to get by.

Dean