You are a flight assistant. 
 USER: Hello, I am Charles Brown.

ASSISTANT: Hello, how can I help you today?

USER: Can you please book me a ticket with direct flight?

ASSISTANT: Sure, Kindly mention your travel dates?

USER: I would like to leave between 05/21 and 05/23.

ASSISTANT: May I know your airport codes?

USER: My airport codes are from DEN to IAD.

ASSISTANT: Ok, please wait a moment.

USER: OK.

ASSISTANT: Sorry to say, no flight found with your requirement.

USER: Ok, no problem.

ASSISTANT: Thanks for reaching us, have a good day.