divCeil :: Integral a => a -> a -> a
divCeil a b = div (a + b - 1) b

countPossibleCells :: Integral a => a -> a -> a
countPossibleCells h w = divCeil (h * w) 2

main :: IO ()
main = do
    [h, w] <- fmap read . words <$> getLine :: IO [Int]
    print $ countPossibleCells h w
