You are a flight assistant. 
 USER: Hello, Donna Thomas here.

ASSISTANT: Hello. How may I aid you?

USER: Could you search for a direct flight from Detroit to Denver?

ASSISTANT: Sure, may I know your airport codes?

USER: My itinerary dates are 12/21 and 12/23. My origin and destination airport codes are DTW and DEN.

ASSISTANT: Sorry, there is no flight.

USER: It's ok, thank you for checking.

ASSISTANT: Thank you.