You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how may I assist you?

USER: I am Lisa Nelson. Can you help me in booking my flight ticket from OAK to DCA?

ASSISTANT: Sure, can you please provide me your travel dates?

USER: My travelling dates will be Aug 26th to Aug 28th.

ASSISTANT: Ok, please wait for a while.

USER: Sure. Make sure that the depart time should be in the evening.

ASSISTANT: I am sorry to tell you that there is no flight available on your preferred travel dates.

USER: Its ok, thank you.

ASSISTANT: You are welcome.