import qualified Data.ByteString.Char8 as BSC8
import qualified Data.Maybe            as Maybe

readIntegers :: IO [Integer]
readIntegers = map ( fst . Maybe.fromJust . BSC8.readInteger ) . BSC8.words <$> BSC8.getLine

main :: IO ()
main = do
  (a:b:_) <- readIntegers
  print $ (a * b + 1) `div` 2
