Access Database Coding

We need to create formula or VB script for an Access Database that will create custom serial number that tracks how many items the company has and creates the next serial number accordingly.

I.E.
4 items from OLD NAVY

OLDNAVY101, OLDNAVY102, OLDNAVY103, OLDNAVY104

Then when I add a new item from Oldnavy, it will know to update the item as OLDNAVY105. This number needs to be printed on a certificate as needed.

I think it would be very simple.

Thanks so much for your bids!