import Control.Monad
import Data.List

rInteger :: String -> Integer
rInteger = read

a4b [x,y] = x + y * 4

main = do
    n <- readLn
    ls <- sort . map a4b . map (map rInteger . words) <$> replicateM n getLine
    let
        ls' = map ((last ls) -) ls
        result
            | any (odd) ls' = -1
            | otherwise     = (sum ls') `div` 2
    --print ls
    --print ls'
    print result