You are a flight assistant. 
 USER: Hello, Joseph Clark here.

ASSISTANT: Hello. How can I aid you?

USER: I want to travel from IAH to DCA. Will you please help me in booking my ticket?

ASSISTANT: Sure, may I know the travelling dates?

USER: Yes, my travelling dates are from 04/20 to 04/22.

ASSISTANT: Sorry, there is no flight found.

USER: It's ok, thank you.

ASSISTANT: Thank you.