You are a flight assistant. 
 USER: Hello, I am Mark Hernandez.

ASSISTANT: Hello, how can I assist you today?

USER: I need to go to Washington, DC - DCA for my exam.Can you please book me flight ticket?

ASSISTANT: Sure, please provide me your travel details?

USER: I need to start on 03/23 from Oakland, CA - OAK and return on 03/25. Make sure that price limit should not cross more than 500 and it should be a direct flight.

ASSISTANT: Ok, please wait a moment.

USER: Sure.

ASSISTANT: Sorry to inform no flight found with your requirement.

USER: Ok, no problem. I will check another option.

ASSISTANT: Thanks for reaching us, have a good day.