import Data.Functor

stride d [] = []
stride d arr = head arr : (stride d $ drop d arr)

main = do
    [n, k] <- map read . words <$> getLine :: IO [Int]
    costs <- map read . words <$> getLine :: IO [Int]
    let d = 2 * k + 1
    print $ maximum $ map (\a -> sum . stride d . drop a $ costs) [0..(k `div` 2)]