module Main where

import Control.Monad (replicateM_)
import qualified Data.Map as M

solve :: String -> Int
solve s =
    if odd $ length s
        then
            (-1)
        else
            sum $ M.filter (> 0) $ M.differenceWith (\a b -> Just (a - b)) m1 m2
  where
    zerocounts = M.fromList $ map (,0) s
    half = length s `div` 2
    s1 = take half s
    m1 = foldl (\m c -> M.insertWith (+) c 1 m) zerocounts s1
    s2 = drop half s
    m2 = foldl (\m c -> M.insertWith (+) c 1 m) zerocounts s2

main :: IO ()
main = readLn >>= flip replicateM_ (getLine >>= print . solve)