import Control.Applicative ((<$>))
import Control.Monad (replicateM)

import Data.ByteString.Char8 (ByteString)
import qualified Data.ByteString.Char8 as B

import Data.List
import Data.Char (isSpace)

main :: IO ()
main = do
  n <- getl $ readi B.readInt
  solve <$> replicateM n (getl $ readil B.readInt) >>= print

solve :: [[Int]] -> Int
solve ab | all even ls || all odd ls = (`div` 2) . sum . map (mx -) $ ls
         | otherwise = (-1)
  where
    ls = map (\[a, b] -> a + 4 * b) ab
    mx = maximum ls
    
getl :: (ByteString -> a) -> IO a
getl f = f <$> B.getLine

readi :: Integral a =>  (ByteString -> Maybe (a, ByteString)) -> ByteString -> a
readi f s = let Just (n, _) = f s in n

readil :: Integral a =>  (ByteString -> Maybe (a, ByteString)) -> ByteString -> [a]
readil f = unfoldr g
  where
    g s = do
      (n, s') <- f s
      return (n, B.dropWhile isSpace s')