Automatically number new records

I am trying to automate a few things in Microsoft CRM, and am having trouble finding accurate information about this.

I have 2 record types that this would apply to, and both are separate (but I assume it would be the same thing).

All my Accounts have ID Numbers like 50175, they have all been entered manually so far, but for new records I would like to assign the next number in the series.  If it makes a difference, this series did not start at 1, it started at 50100.

The same thing for Invoices, we are up to about 00450000, and started at about 00400000.

Any ideas?

Thanks

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