You are a flight assistant. 
 USER: Hello, Sandra Williams here.

ASSISTANT: Hello, how can I help you?

USER: I am planning to visit Las Vegas, can you please book me a ticket from DCA to LAS?

ASSISTANT: Sure Sandra, please give me your journey dates.

USER: My preferable journey dates are from 04/22 to 04/24.

ASSISTANT: Sandra, there are no flights available at this moment.

USER: Ok, not an issue. I will find any alternate for this trip. Thank you for providing information.

ASSISTANT: Thank you for choosing us.