You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how can I aid you?

USER: I want to travel from LAS to DFW. Can you help me to book a flight ticket?

ASSISTANT: Sure, can you share your trip dates?

USER: I want to travel in the flight on Dec 16 and Dec 18.

ASSISTANT: Do you have any preferences?

USER: I want to depart in evening time.

ASSISTANT: May I know your name?

USER: I am Rachel Martin.

ASSISTANT: Sorry, there is no flight on your given dates.

USER: That's ok. Anyways, thank you for your help.

ASSISTANT: Thank you for opting us.