main = getLine >>= print . solve' . solve . map read . words
        where solve' n
                | n < 0 = 0
                | otherwise = n

solve :: [Int] -> Int
solve [l, k]
        | m == 0 = h - k
        | otherwise = h - m
        where
        h = div l 2 + if odd l then 1 else 0
        m = mod h k