Page numbers in form field

I want to use the page number in a Javascript formula in a form field, but can‘t get it to work properly. I am using the script:

event.value=pageNum;

but this gives the page number of the last field that was changed! So in a multiple page document with the same script on each page, the result is the same on every page! I need it to always return the page number of the page on which the formula is. How can I do this? Or could I set a hidden footer, and reference it in a script?