You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How can I  help you?

USER: I am Mark Lee. Can you please book me a ticket from DCA to SFO?

ASSISTANT: Your travelling dates please?

USER: My planned trip dates are May 26th and 28th.

ASSISTANT: I am sorry no flights found on your route.

USER: Ok, that's fine.

ASSISTANT: Thank you for choosing us.

USER: Anyways, thanks for the information.