import Data.List
main = do
    h:w:other <- (take 2).read.words<$>getLine
    print $ f h w
    	where 
        	f x 0 = 0
            f x y = ((div x 2) + (mod x 2)) + g x (y-1)
            g x 0 = 0
            g x y = (div x 2) + f x (y-1)