import System.IO
import Control.Monad

main :: IO ()
main = do

    input_line <- getLine
    
    let numberlist = map read ( words input_line ) :: [Int]
    let answer = ( ( numberlist !! (1) ) * ( numberlist !! (0) ) + 1 ) `div` 2 - 1
    

    putStrLn $ show answer
    return ()