You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: Lisa Rodriguez here.

ASSISTANT: How can I help you?

USER: I have some urgent work in Houston, could you book my ticket in a direct flight?

ASSISTANT: Sure, I will help you. May I know your desired travel dates please?

USER: My itinerary dates are 07/12 and 07/14.

ASSISTANT: Can I know your preferred airport codes please?

USER: I will depart from DEN and land at HOU.

ASSISTANT: Please wait a while.

USER: Sure.

ASSISTANT: Sorry to say, there is no flight on your desired travel dates.

USER: It's ok, thank you.

ASSISTANT: Thank you for opting us. Have a nice day.