It's on me.

Ron makes a phone call to the restaurant to reserve a table for him and his friends for dinner....
	Mary:	Thanks for lunch. It was delicious.
	Shelly:	It's OK.
	Mary:	Next time lunch is on me.
	Shelly:	Don't be silly.
	Mary:	I'm serious.
	Shelly:	All right. Next time you'll treat.
	Mary:	It's a deal!