Formula Help - Generate new record number if name doesn't exist

[susanmurray]

I am working on creating a membership database in Excel. I have separate pages for Memberships, Donations, Sponsorships, and Receipts (to be entered as we get them).
I am trying to set up a formula, that matches the record number of a Donor when their name is entered into receipts. If they are a new entry, and their name does not already exist on the donor list, then I want to assign a new number to them.

So far, I have used:
=INDEX(Donors!A:A,MATCH(1,(C2=<wbr>Donors!B:B)*(D2=Donors!C:C),0)<wbr>)
to return the record number of a name already in the system - I am not sure how to manipulate this to assign a new number if the name does not exist.
On Donors Spreadsheet:
A B C
1 John Smith
2 Jerry Jones

On Receipts Spreadsheet
A B C
1 (formula) John (manual) Smith (manual)
#N/A (formula)Fred (Manual) Martin (Manual)

What formula do I use to turn #N/A into "3"
I am using Excel 2007.
Many Thanks
Sue