Question: who signed the treaty to purchase alaska from russia
[{'answer': 'Andrew Johnson', 'provenance': [{'wikipedia_id': '193703', 'title': 'Alaska Purchase', 'start_paragraph_id': 1, 'start_character': 261, 'end_paragraph_id': 1, 'end_character': 275, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '193703', 'title': 'Alaska Purchase', 'start_paragraph_id': 1, 'start_character': 39, 'end_paragraph_id': 1, 'end_character': 275, 'bleu_score': 0.5752434330407, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 6899534962723020529}, 'section': 'Section::::Abstract.'}]}]