Question: when was the 50th star added to the united states flag
[{'answer': '1960'}, {}, {'provenance': [{'wikipedia_id': '8872619', 'title': 'Timeline of the flag of the United States', 'section': 'Section::::Abstract.', 'start_paragraph_id': 45, 'start_character': 12, 'end_paragraph_id': 45, 'end_character': 37, 'bleu_score': 1.0}]}]