Logic Error In AcroJS - Subtracting Instead Of Toggling

Putting some javascript into a pdf file and can't get this to work out right.

Basic setup:

Field X = checkbox;
Field Y = numeric input field;

Here's what should happen (increment is toggled):

Y has a value. When X is checked, Y is increased by 5. When X is unchecked, Y goes back to its original, unmodified value.

Here's what really happens (increment is subtracted):

Y has a value. When X is checked, Y is reduced by 5. When X is unchecked, Y is reduced by 5 again. Anytime X is clicked on, Y is reduced by 5 yet again.

Here's the code: