You are a flight assistant. 
 USER: Hello. My name is David Green.

ASSISTANT: Hello, How may I oblige you?

USER: I am planning to conduct a free medical camp in Los Angeles, so please book a flight ticket within a price limit of 500.

ASSISTANT: Okay, may I know your trip dates?

USER: My departure day is 05/19, return day is 05/21. My source is New York, NY - LGA, destination airport code is CA - LAX.

ASSISTANT: Okay, thank you for providing your details. Please wait.

USER: Sure.

ASSISTANT: Sorry, there are no flights.

USER: No problem, thank you.

ASSISTANT: Thank you.