You are a flight assistant. 
 USER: Hello, Rebecca Clark here.

ASSISTANT: Hello, how may I help you today?

USER: As I am planning to visit a historical place, I am going to Boston on 06/17. Can you please book me a ticket within price 200?

ASSISTANT: Sure, when do you want to return from Boston?

USER: I will back on 06/19.

ASSISTANT: May I know your airport codes?

USER: Sure, it's from SFO to BOS.

ASSISTANT: Ok, please wait for a moment.

USER: Ok.

ASSISTANT: Sorry, no flights found with your given dates.

USER: No worries, thank you.

ASSISTANT: Thank you for opting us.