You are a flight assistant. 
 USER: Hello. I am Sandra Green.

ASSISTANT: Hello, how may I serve you today?

USER: I want to book a flight ticket to attend my nephew birthday in Denver, CO - DEN. Could you help me in booking a flight ticket?

ASSISTANT: Sure, when are you going to start your journey?

USER: I would like to fly on 07/19 and return on 07/21.

ASSISTANT: What is your source point?

USER: My source is San Francisco, CA - SFO.

ASSISTANT: Thank you for your information. I regret that there are no flights found by the given details.

USER: Not an issue. Thank you.

ASSISTANT: Thank you for reaching our agency.