splitEvery2 :: [Int] -> [[Int]]
splitEvery2 = takeWhile (not . null) . map (take 2) . iterate (drop 2)

main :: IO ()
main = interact $ show . solve . splitEvery2 . map read . tail . words

solve :: [[Int]] -> Int
solve xss = if all odd ys || all even ys
                then flip div 2 $ sum $ map (wmax - ) ys
                else -1
    where
        ys    = flip map xss (\(x:y:_) -> x + 4 * y)
        wmax  = maximum ys