# C, 30 bytes
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[**Try Online**][1] 1-indexed

    s;f(n){s=n>2?f(n-2)-f(n-1):n;}


  [1]: https://tio.run/##FcsxCoAwDEDRq4ggJLSFqpuxehapVDIYpXETz151@m/50W0xlqKUQPDWIFM3f3Qduj8tDkJPYdoXFsA7HRk4tMRj74mNsRo8JmC0Z2a5EtTNaqvaKn7XCw