Page Number based on slice() method

I have a document that is spawning pages. So for simplicity with spawning renaming pages, I decided I would just use the slice() method to obtain the page number based on the spawing pages:

P3.mytemplate

var p_name = event.target.name;

var ram = p_name.slice(1,2);

event.value = ram;

Now this works great until the last page and I get: NaN filled in the text field. If I delete this then the number shows up correctly and stays. Any ideas what might be causing this??

Thanks as always,

Nathan