Funny Little Question.

Hi :) Just registered and was wondering if anyone could help me out.

I have a very simple table for which I am making a form. The table has two values:

'Week Number' (Number) and 'Period' (Number)

What I need is for the 'Period' field to increment with every new record created (easy enough) but when the week number changes I would like the 'Period' number to reset back to 1 in the following fashion:

Week||Period
...1.........1
...1.........2
...1.........3
...1.........4
...1.........5
...1.........6
...1.........7
...1.........8
...2.........1
...2.........2
...2.........3
...3.........1
...3.........2
...3.........3
...3.........4

etc...

If anyone can help me it would be greatly appreciated, thanks :)

Pob