Question: when was the first casino opened in las vegas
[{'answer': '1931', 'provenance': [{'wikipedia_id': '447621', 'title': 'Las Vegas Strip', 'start_paragraph_id': 12, 'start_character': 71, 'end_paragraph_id': 12, 'end_character': 75, 'bleu_score': 1.0, 'section': 'Section::::History.:Early years (1930s–1990s).\n'}]}, {'provenance': [{'wikipedia_id': '447621', 'title': 'Las Vegas Strip', 'start_paragraph_id': 12, 'start_character': 0, 'end_paragraph_id': 12, 'end_character': 757, 'bleu_score': 0.9625354647617571, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 7480868179710752748}, 'section': 'Section::::History.:Early years (1930s–1990s).\n'}]}]