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-- https://blog.miz-ar.info/2019/01/fast-fibonacci/
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fastDoubling :: Int -> (Integer, Integer)
fastDoubling 0 = (0, 1)
fastDoubling 1 = (1, 1)
fastDoubling i
  = let (a, b) = fastDoubling (i `quot` 2)
    in if even i
       then (a * (2 * b - a), a * a + b * b)
       else (a * a + b * b, b * (2 * a + b))

fib :: Int -> Integer
fib i = case fastDoubling i of
          (a, _) -> a
main :: IO ()
main = readLn >>= print . fib . succ