You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how may I serve you at this moment?

USER: I am Justin Turner, I would like to travel from Dallas to attend a business meeting in San Francisco, can you help me to make a direct flight reservation?

ASSISTANT: Sure, I will help you. Please send your travelling dates.

USER: My travelling dates are June 13 and June 15.

ASSISTANT: Please wait for a moment, do you have any time preference?

USER: No, any time is comfortable for me.

ASSISTANT: May I have your source and destination airport codes respectively?

USER: Sure, I would like to travel from DFW to SFO.

ASSISTANT: Please wait for a moment.

USER: I will wait for your response.

ASSISTANT: Sorry, there is no flight available on your requested preferences.

USER: That's fine, thanks for your information.

ASSISTANT: You are welcome, have a nice day.