[AccessD] Assign Document Numbers

I've created a database for use by a group of individuals (perhaps 20 users)
in which the user will open a form and place a request for a document
number. I need a document number to be automatically assigned at that time.
 
An example of document numbers and their scheme are as follows: 
 
DC-2101-0001
DC-2204-0001
DC-2301-0001
LA-2605-0001
LA-2605-0002
LA-2605-0003
NR-2605-0001
 
It's the last set of numbers (0001, 0002, 0003) that I wish to have assigned
according to which first two sets (DC) and (2101) are input by the user.
Like if a user inputs "DC" and "2101"....I'd like the system to
automatically assign the next available number which would be "0002".
Therefore the document number assigned would be "DC-2101-0002".
 
Also, the bad thing for me is that we have already been using this database
(just one individual) and manually assigning these numbers. I don't know how
it would work out if we suddenly put the code in. Would it acknowledge
previous entries and assign the correct number?
 
If someone can help me with this, I'd be eternally grateful.
 
Virginia
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