Question: how many manga books are there for attack on titan
[{'answer': '26', 'provenance': [{'wikipedia_id': '39101492', 'title': 'List of Attack on Titan chapters', 'start_paragraph_id': 9, 'start_character': 202, 'end_paragraph_id': 9, 'end_character': 204, 'bleu_score': 1.0, 'section': 'Section::::Manga.:"Before the Fall".\n'}]}]