As you probably booked the 5th night free, 10 nights will cost you the same as 8 nights, not sure if you will shorten your stay at end or beginning of your trip.
Contact the hotel and let them know about your arrival/departure dates. They suggest to book additional nights to secure early check in/late check out.
Rangali will also get money for a 10 nights, instead of 8 nights from Hilton.