{-# LANGUAGE BangPatterns #-}
import           Control.Monad.ST
import qualified Data.Vector.Fusion.Stream.Monadic as VFSM
import qualified Data.Vector.Unboxed               as VU
import qualified Data.Vector.Unboxed.Mutable       as VUM

main :: IO ()
main = readLn >>= print . fib

fib :: Int -> Int
fib n = runST $ do
  ret <- VUM.unsafeNew (n + 1)
  VUM.unsafeWrite ret 0 (1 :: Int)
  VUM.unsafeWrite ret 1 (1 :: Int)
  rep n $ \i -> do
    a <- VUM.unsafeRead ret (i - 2)
    b <- VUM.unsafeRead ret (i - 1)
    VUM.unsafeWrite ret i (a + b)
  return =<< VU.last <$> VU.unsafeFreeze ret

stream :: Monad m => Int -> Int -> VFSM.Stream m Int
stream !l !r = VFSM.Stream step l
  where
    step x
      | x < r     = return $ VFSM.Yield x (x + 1)
      | otherwise = return $ VFSM.Done
    {-# INLINE [0] step #-}
{-# INLINE [1] stream #-}

rep :: Monad m => Int -> (Int -> m ()) -> m ()
rep n = flip VFSM.mapM_ (stream 2 (n + 1))
{-# INLINE rep #-}