import Control.Applicative ((<$>), (<*>))
import Data.List (transpose, foldl', sort)
import Data.List.Split (chunksOf)

main :: IO ()
main = solve <$> f <*> f >>= print
  where f = map read <$> words <$> getLine

solve :: [Int] -> [Int] -> Int
solve [_, k] = g . foldr f (0, []) . transpose . chunksOf k
  where f xs (ct, ls) = let (ct', ls') = (swapCount xs, sort xs)
                        in (ct + ct', ls' : ls)
        g (ct, xs) | (isSorted . concat . transpose) xs = ct
                   | otherwise = -1

swapCount = fst . foldl' f (0, [])
  where f (ct, ys) x = let ct' = length . filter (x<) $ ys
                       in (ct + ct', x:ys)

isSorted :: [Int] -> Bool
isSorted xs = and $ zipWith (<) xs (tail xs)