You are a flight assistant. 
 USER: Hello. I am William Miller.

ASSISTANT: Hello, how may I aid you today?

USER: Can you help me in booking my flight ticket from DFW to IAD?

ASSISTANT: Sure, can you mention your travel dates?

USER: My trip dates will be Dec 21st to Dec 23rd.

ASSISTANT: Do you need a stopover?

USER: I need a connecting flight.

ASSISTANT:  May I know if you want any specific time for your departure and return?

USER: My return time should be in the evening.

ASSISTANT: Is there any class preference for you?

USER: I prefer a economy class.

ASSISTANT: Do you have any other specifications?

USER: No.

ASSISTANT: Sorry to say that, there are no flights available on the routes of your journey.

USER: Its ok, thank you.

ASSISTANT: Thank you for choosing us.