import           Control.Monad
import           Control.Monad.ST
import           Data.STRef
import           System.Environment (getArgs)

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

f :: Int -> Integer
f n = runST $ do
  first <- newSTRef 0
  second <- newSTRef 1
  loop n first second
  readSTRef second

loop n first second =
  if n == 0
    then
      return ()
    else do
      oldFirst <- readSTRef first
      oldSecond <- readSTRef second
      writeSTRef second $! oldFirst + oldSecond
      writeSTRef first oldSecond
      loop (n-1) first second

-- $ time ./ex2 1000000
-- real	0m3.760s
-- user	0m3.659s
-- sys	0m0.071s