You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how can I help you today?

USER: My name is Gary Wright. I want to celebrate Christmas in Dallas, so please book a flight ticket with a price limit of 500.

ASSISTANT: Sure, kindly provide me your travel dates?

USER: My departure day is 12/25, return day is 12/27.

ASSISTANT: May I know your airport codes?

USER: My source and destination airport codes are TX-HOU, TX-DFW.

ASSISTANT: Ok, please wait a moment.

USER: Ok.

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

USER: No problem, thanks for your help.

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