import Control.Applicative ((<$>))

solve :: Integral a => a -> a -> a
solve r c
    | odd r && odd c && r == c = flip div 4 $ prod - 1
    | odd r && odd c           = flip div 2 $ prod - 1
    | r == c                   = subtract 1 $ flip div 4 $ prod
    | otherwise                = subtract 1 $ flip div 2 $ prod
    where prod = r * c

main :: IO ()
main = do
    [r, c] <- map read . words <$> getLine
    print $ solve r c