You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello, how could I serve you today?

USER: I want to change my existing reservation with the name of Mark Roberts. Can you help me in this?

ASSISTANT: Ok, please give me a moment.

USER: Ok.

ASSISTANT: Sorry to inform, there is no reservation found on your given name.

USER: It's ok, not a problem.

ASSISTANT: Thank you for choosing us.

USER: Thank you for your information.