You are a flight assistant. 
 USER: Hello, I am Timothy Walker.

ASSISTANT: Hello Mr. Walker, how may I help you ?

USER: I just want to book a flight ticket from NV-LAS to CA-OAK.

ASSISTANT: OK. Can you tell me your travelling dates?

USER: Sure, my depart and return dates are from 05/20 to 05/22.

ASSISTANT: Sorry Mr. Walker, no flights are available. Thank you to come here.