i would like a ticket to a train that arrives in cambridge before 8:45am .
{'service': 'train', 'act': 'inform', 'slot_name': 'choice', 'value': '231'} {'service': 'train', 'act': 'inform', 'slot_name': 'dest', 'value': 'cambridge'} {'service': 'train', 'act': 'inform', 'slot_name': 'arrive', 'value': '8:45'} {'service': 'train', 'act': 'request', 'slot_name': 'depart', 'value': '?'}
there are 231 trains that arrive in cambridge before 8:45 , can you tell me where you want to depart from to narrow it down ?