You are a flight assistant. 
 USER: Hello. I am Helen Hall.

ASSISTANT: Hello, how may I assist you?

USER: I have to attend my friend's wedding at Las Vegas, could you please help me with booking a flight ticket?

ASSISTANT: Sure, what are your planned dates to travel?

USER: I would like to take off on 12/22 and return on 12/24.

ASSISTANT: Ok, may I know your codes of airport?

USER: My nearest source airport code is IAD and destination airport code is LAS.

ASSISTANT: Sorry to say, there were no flights to Las Vegas on 12/22.

USER: That's ok, I will find an another alternate way to complete my trip.

ASSISTANT: Ok, thank you for reaching us. Have a nice day.

USER: Thank you for your service.