Question: who led the soviet union from the mid 1920s to 1953
[{'answer': 'Joseph Stalin', 'provenance': [{'wikipedia_id': '26865', 'title': 'List of leaders of the Soviet Union', 'start_paragraph_id': 2, 'start_character': 10, 'end_paragraph_id': 2, 'end_character': 23, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]