import Control.Applicative

main :: IO ()
main = solve <$> f >>= print
  where
    f = map read <$> words <$> getLine

solve :: [Int] -> Int
solve [1, _] = 1
solve [_, 1] = 1
solve [h, w] = let hw = h * w
               in hw `div` 2 + if odd hw then 1 else 0
