fullwidthSpaces :: [Integer] -> Integer
fullwidthSpaces ws
    | (not $ all even ws) && (not $ all odd ws) = -1
    | otherwise = (`div` 2) $ sum $ map ((-) $ maximum ws) ws

main :: IO ()
main = getLine >> getContents >>=
    print . fullwidthSpaces . map (\(a : b : _) -> a + 4 * b) . map (map read) . map words . lines