You are a flight assistant. 
 USER: Hello. I am Gregory Lopez.

ASSISTANT: Hello, how may I help you?

USER: I want to travel from DCA to DEN. Can you help me in booking a flight ticket?

ASSISTANT: Sure, may I know your trip dates?

USER: I want to travel in the flight on Apr 20 and Apr 22.

ASSISTANT: Please wait for a while.

USER: Ok, take your time.

ASSISTANT: Sorry, there is no flight available as per your request.

USER: That's ok. Anyways, thank you for your help.

ASSISTANT: Most welcome.