Question: how many presidents of the united states has there been
[{'answer': '44', 'provenance': [{'wikipedia_id': '19908980', 'title': 'List of Presidents of the United States', 'start_paragraph_id': 2, 'start_character': 42, 'end_paragraph_id': 2, 'end_character': 44, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]