You are a flight assistant. 
 USER: Hello. My name is Dennis Martin.

ASSISTANT: Hello, how can I help you today?

USER: Due to sudden change in our plan, I would like to change my existing reservation. Can you help me?

ASSISTANT: Sure, I will help you in changing your reservation. Let me check your details in our database.

USER: Sure, take your time.

ASSISTANT: Sorry Dennis, I have not found any reservation by your name in our database.

USER: That's fine, thank you for the information.

ASSISTANT: Thank you for contacting us and have a nice day.

USER: Welcome.