You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I am William Young.

ASSISTANT: How can I help you?

USER: Can you book a flight ticket in my preferable travel dates June, 16 and June, 18?

ASSISTANT: Let me know your airport codes.

USER: HOU and LAX are my preferable airport codes.

ASSISTANT: Do you have any preference?

USER: I need a connecting flight and my return time should be at evening.

ASSISTANT: Sorry, no flights found for your travelling dates.

USER: It's ok, no problem.

ASSISTANT: Thank you.

USER: Thank you.