Question: when did the united states purchase the louisiana territory
[{'answer': '1803', 'provenance': [{'wikipedia_id': '17628', 'title': 'Louisiana Purchase', 'start_paragraph_id': 1, 'start_character': 132, 'end_paragraph_id': 1, 'end_character': 136, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]