Question: what year did the beatles first perform in the united states
[{'answer': '1964', 'provenance': [{'wikipedia_id': '844475', 'title': 'Beatlemania', 'start_paragraph_id': 1, 'start_character': 335, 'end_paragraph_id': 1, 'end_character': 339, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]