import Data.IntMap ((!))
import qualified Data.IntMap as M
import Control.Monad
import Data.List

apply n f x = foldr ($) x (replicate n f)

splitOn n = unfoldr (\xs -> guard ((not.null) xs) >> return (splitAt n xs))

nBubble xs = fst $ apply (length xs - 1) replace (0,ixs)
 where
  replace (n,im) = foldr (\i acc -> swap i (i+1) acc) (n,im) [1 .. length xs - 1]
  ixs = M.fromList (zip [1..] xs)

swap i j (n,im)
 | im!i > im!j = (n+1, M.insert i (im!j) (M.insert j (im!i) im))
 | otherwise = (n,im)

main = do
 [_,k] <- map read . words <$> getLine
 as <- map read . words <$> getLine :: IO [Int]
 putStrLn $ maybe "-1" show (rsort k as)

rsort k as = do
 let ms = transpose $ splitOn k as
 let sms = map sort ms
 guard (concat (transpose sms) == sort as)
 return $ sum (map nBubble ms)