main :: IO ()
main = do

    input_line <- getLine
    
    let numberlist = map read ( words input_line ) :: [Int]
    let a = numberlist !! (0)
    let b = numberlist !! (1)
    
    
    let answer  | a == b && odd a  = ( ( a * a ) - 1 ) `div` 4 
                | a == b && even a = ( a * a ) `div` 4 - 1
                | a /= b && odd a && odd b = ( a * b - 1 ) `div` 2
                | otherwise = ( a * b ) `div` 2 - 1
    

    putStrLn $ show answer
    return ()