import Control.Applicative

main = do
	n <- readLn
	wid <- map ((\(a:b:_) -> a + 4 * b) . map read . words) . lines <$> getContents
	let sumOfModTwo = sum (map (`mod` 2) wid)
	let max = maximum wid
	print $
		if sumOfModTwo == n || sumOfModTwo == 0
			then sum [(max - x) `div` 2|x <- wid]
			else -1