import           System.Environment (getArgs)

main :: IO ()
main = do
  n <- fmap (read . head) $ getArgs
  putStrLn . show $ f n


f :: Int -> Int
f n =
  if n <= 2
    then n
    else
      f (n-1) + f (n-2)

-- $ time ./ex1 40
-- real  0m10.497s
-- user  0m10.416s
-- sys   0m0.061s