You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how can I help you?

USER: I am Sandra Davis. Can you book please a ticket on 12/18?

ASSISTANT: Sure, when will you return date?

USER: Return is on 12/20.

ASSISTANT: Please share your airport codes.

USER: Source is BOS and destination is DEN.

ASSISTANT: Please standby.

USER: Sure.

ASSISTANT: Thanks for waiting, there are no flights available on your specified dates.

USER: No problem. Thank you.

ASSISTANT: Thank you for contacting us.