Given the task definition and input, reply with output. In this task, you are given a conversation between a flight agent and the customer. You are given 4 options and you need to select the goal of the conversation. It is provided as part of customer's context, which has to be one of the following: `book`: Make a new reservation, `cancel`: Cancel an existing reservation, `no_flight`: No flight found for the customer's preference, `no_reservation`: No reservation / No change / No cancellation was made in the conversation, mostly enquiring details

customer: Hello. 
 agent: Hello. How can I help you? 
 customer: My name is Stephen Johnson. As my business trip got postponed to other dates, I would like to change my existing reservation. 
 agent: Please wait for a while. 
 customer: Sure. 
 agent: Sorry, there is no reservation found by your name. 
 customer: That's fine. Thank you for your information. 
 agent: Thank you and have a nice day.
no_reservation