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 may I assist you? 
 customer: Can you help me to book a flight ticket from OAK to LAX? 
 agent: Sure, I will help you. May I know your planned journey dates please? 
 customer: Ok, please for the dates Sept 3rd and Sept 5th respectively. 
 agent: Can I have your name please? 
 customer: Yes, my name is Melissa Perez. 
 agent: Do you have any connection preference? 
 customer: Ok, please book a connecting flight. 
 agent: May I know your class preference? 
 customer: Yes, I need a economy class flight. 
 agent: Do you have any other preferences? 
 customer: Ok, please check for a flight which depart during evening. 
 agent: Melissa Perez, no flights has been found on your specifications. 
 customer: Ok, that is not a problem. Thank you for assisting me. 
 agent: Thank you for choosing us.
no_flight