You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello. How can I help you?

USER: I am looking for a direct flight from IAD to SFO, can you please help me in booking a flight ticket?

ASSISTANT: Let me know your travelling dates.

USER: My travelling dates are Sept 18 and Sept 20.

ASSISTANT: Do you have any preference?

USER: My class preference is business.

ASSISTANT: Can I know your name?

USER: My name is Margaret Lee.

ASSISTANT: Sorry, no flights found for your travelling dates.

USER: No problem, thank you for your information.

ASSISTANT: Thank you.