Is this a good idea?

I have a book signing coming up soon and I will be signing my 4th and 5th books that recently came out- But I want to keep some interest in the first book which the 1st of a four book series.
My thought was (if Barnes and Nobles allows this) to give a copy of my first book to the first 20 people who buy either of the books I am signing.
That way if they like the first- perhaps they might buy the rest of the series- I would bring the free books- B&N would have no expense there- so is this a good idea or a bad one?

Thanks Ken for the opinion- I see no down side to it either- but wanted to throw it out there to the masses-

Thanks for the advice- Yeah- I did check with them and while they have the two I am signing along with the second and third in my series they do not currently have the first ( the one I want to give away) The events coordinator just got back to me and said it would be fine- so cool!

Thanks! I'll let you know how it works out!