import qualified Data.ByteString.Char8 as BS
import Data.Maybe

tuplify2 (x : y : _) = (x, y)
tuplify2 _ = undefined

readInt = fst . fromJust . BS.readInt

readIntTuple = tuplify2 . map readInt . BS.words

getIntTuple = readIntTuple <$> BS.getLine

main = getIntTuple >>= \xs -> print $ maximum [0, fst xs - snd xs * 2]
