Question: when did the boston celtics win their first championship
[{'answer': '1957', 'provenance': [{'wikipedia_id': '43376', 'title': 'Boston Celtics', 'start_paragraph_id': 3, 'start_character': 279, 'end_paragraph_id': 3, 'end_character': 283, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '43376', 'title': 'Boston Celtics', 'start_paragraph_id': 12, 'start_character': 0, 'end_paragraph_id': 12, 'end_character': 1230, 'bleu_score': 0.9254029143425531, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8861996039742171624}, 'section': 'Section::::Franchise history.:1957–1969: The Bill Russell era.\n'}]}]