import Control.Applicative ((<$>))

main :: IO ()
main = solve <$> map read <$> words <$> getLine >>= print
  
solve :: [Int] -> Int
solve [r, c] | r == c = rc `div` 4 - if even rc then 1 else 0
             | otherwise = rc `div` 2 - if even rc then 1 else 0
  where rc = r * c