Which Roman emperor served the longest, in years?
{'directed': True, 'multigraph': False, 'graph': {}, 'nodes': [{'type': 'INTERNAL', 'name_': 'Q1416', 'id': 0, 'label': 'Otho'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q842606', 'id': 1, 'label': 'Roman emperor'}, {'type': 'ANSWER_CANDIDATE_ENTITY', 'name_': 'Q1747689', 'id': 2, 'label': 'Ancient Rome'}], 'links': [{'name_': 'P39', 'source': 0, 'target': 1, 'label': 'position held'}, {'name_': 'P27', 'source': 0, 'target': 2, 'label': 'country of citizenship'}]}