import Data.Array.IO

rev :: IOUArray Int Int -> IO ()
rev ary = do
  (l,r) <- getBounds ary
  rev' ary l r ((r - l + 1) `div` 2) 0
  return ()
  where
    rev' ary init end len n
      | n < len = do
          let (a,b) = (init + n, end - n)
          x <- readArray ary a
          y <- readArray ary b
          writeArray ary a y
          writeArray ary b x
          rev' ary init end len (n+1)
      | otherwise = return ()

size = 1000000
main = do
  ary <- newListArray (1, size) [1..size] :: IO (IOUArray Int Int)
  rev ary
  print =<< readArray ary 1