Question: how many series are there of boardwalk empire
[{'answer': '5', 'provenance': [{'wikipedia_id': '24565032', 'title': 'Boardwalk Empire', 'start_paragraph_id': 2, 'start_character': 226, 'end_paragraph_id': 2, 'end_character': 227, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]