import Control.Applicative

main = do
	a <- readLn
	print $ maybe $ foldl1 func $ map (func2 a) [2..100]
		where
			Nothing `func` Nothing = Nothing
			Nothing `func` (Just x) = Just x
			(Just x) `func` _ = Just x
			func2 a x
				| (x + 7) == a = Just x
				| otherwise = Nothing
			maybe (Just x) = x
			maybe Nothing = -1