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

aku r c
 | r==c && even r = div (r*c) 4 - 1
 | r==c && odd r = div (r*c) 4
 | even (r*c) = div (r*c) 2 - 1
 | otherwise = div (r*c) 2