Looking to add a 2nd Server 2008 to existing domain run on server 2008, just for RDS role... do I need RDS CALs as well?

I have an existing Domain with a single Server 2008 R2 Std install, carrying all the usual roles and 15 clients.

Several clients now wish to work from home, and I am wanting to add a 2nd Server 2008 R2 box to the Domain, specifically for the RDS role.

My question is... do the 5 CALs that come with 2008 R2 cover off the RDS, or do I still need to buy 5 RDS CALs as well.

Thanks