Auto-incrementing Number That Starts Over

I have a table, Purchases, which has a 1:N relationship with another table, Items. Each purchase can have multiple Items.

'PurchaseID' is the primary key of my Purchases table.

The Items table has a composite primary key, made up of PurchaseID (making it an identifying relationship) and ItemID.

As you can see, I have ItemID starting reverting back to 1 each time PurchaseID increments. PurchaseID is an autonumber on the Purchases table, however ItemID is just a Number and has to be entered manually. If I set ItemID to an autonumber, it will continue to increment even after PurchaseID changes.

What I am trying to do is to get ItemID to auto-increment but revert back to 1 each time PurchaseID increments.

The entry form for both looks like this:

The Record Source of the main form is the Purchases table, and the subform is a Multiple Items Form based on the Items table. Filling out the form will create a new Purchase record with an auto-incremented PurchaseID, and a number of new Item records with that same PurchaseID, and whatever ItemID's are specified in the subform. Currently the ItemID's have to be entered manually as 1, 2, 3, etc.

Is there a way to get them to auto-increment, but begin from 1 for each new Purchase record?