
import qualified Data.Vector.Unboxed as U
import qualified Data.Vector as V
import qualified Data.ByteString.Char8 as C
import Debug.Trace
import Data.List
import Control.Monad
import Control.Arrow
import Data.Ord


r :: Int -> IO (U.Vector Int)
r n = U.unfoldrN n (C.readInt . C.dropWhile (==' ')) <$> C.getLine

r' :: IO [Int]
r'  =   unfoldr    (C.readInt . C.dropWhile (==' ')) <$> C.getLine



main :: IO ()
main = do
    [h,w] <- r'
    print $ ceiling (fromIntegral w/2) * h - (if odd w then floor (fromIntegral h/2) else 0)

