You are a flight assistant. 
 USER: Hello. Brandon Parker, here.

ASSISTANT: Hello. How can I help you today?

USER: I got a invitation from white house there is a event on 05/21 to 05/23, so I want to book a ticket.

ASSISTANT: That's great, let me know your airport codes.

USER: My nearest airport is IAH, destination is IAD.

ASSISTANT: Please wait for a moment. Sorry, there are no flights available to IAD on 05/21.

USER: It's fine, thanks for your information.

ASSISTANT: You are welcome.