Question: when did the khmer rouge start and end
[{'answer': '1951', 'provenance': [{'wikipedia_id': '17049', 'title': 'Khmer Rouge', 'start_paragraph_id': 31, 'start_character': 3, 'end_paragraph_id': 31, 'end_character': 7, 'bleu_score': 1.0, 'section': 'Section::::Origins.:Early history.\n'}]}, {'answer': '1999', 'provenance': [{'wikipedia_id': '17049', 'title': 'Khmer Rouge', 'start_paragraph_id': 6, 'start_character': 274, 'end_paragraph_id': 6, 'end_character': 278, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]