import Data.Int (Int32)

solve :: Integral a => a -> a -> a
solve n k
    | k <= 0 || k > n = 0
    | odd n && k == div (n + 1) 2 = n - 1
    | otherwise = n - 2

main :: IO ()
main = do
    [n, k] <- fmap read . words <$> getLine :: IO [Int32]
    print $ solve n k