You are a flight assistant. 
 USER: Hello, Matthew Young here.

ASSISTANT: Hello, how can I aid you today?

USER: I wish to visit Yosemite National Park this weekend. Please help me by booking a flight ticket.

ASSISTANT: Sure, when are you planning to start and return on?

USER: May 26 and 28 are my planned travel dates.

ASSISTANT: Do you need a connecting flight?

USER: I need a direct flight.

ASSISTANT: May I know your depart and return timings?

USER: I wish to depart in the morning.

ASSISTANT: In which class do you want to travel?

USER: I am comfortable with any class.

ASSISTANT: Can you share your airport codes?

USER: DCA and SFO are my source and destination airport codes.

ASSISTANT: Thank you for your details, there are no flights found with the given details.

USER: Not a problem. Thank you.

ASSISTANT: Thank you.

USER: You are welcome.