{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE BinaryLiterals #-}
{-# LANGUAGE BangPatterns #-}

-- Default Imports

import Control.Monad
import Control.Monad.ST
import Control.Monad.Trans.Class

import qualified Control.Monad.Reader as Reader
import Control.Monad.Reader (Reader, ReaderT, runReader, runReaderT)

import qualified Data.Array.MArray as MArray
import qualified Data.Array.ST as STArray
import Data.Array.ST (STArray, STUArray, runSTArray, runSTUArray)
import Data.Ix (Ix)

import qualified Data.Bits as Bits
import Data.Bits ((.&.), (.|.))

import qualified Data.ByteString.Char8 as BS
import Data.ByteString.Char8 (ByteString)

import qualified Data.Char as Char
import qualified Data.List as List
import qualified Data.Map as Map
import Data.Map (Map)
import qualified Data.Maybe as Maybe
import qualified Data.Set as Set
import Data.Set (Set)

import qualified Data.Vector as Vector
import qualified Data.Vector.Generic as GVector
import qualified Data.Vector.Mutable as MVector
import Data.Vector.Mutable (STVector)
import qualified Data.Vector.Unboxed as UVector

import Numeric

import Debug.Trace
-- ----------

-- Custom Import Here


-- ----------

-- Default Definitions

type Vector = Vector.Vector
-- type GVector = GVector.Vector
type UVector = UVector.Vector

readIntToList :: ByteString -> [Int]
readIntToList = List.unfoldr (fmap (second $ BS.dropWhile Char.isSpace) . BS.readInt)

readToList :: Read a => ByteString -> [a]
readToList = List.unfoldr $ fmap (mapTaken . mapRemain) . span' (not . Char.isSpace)
	where
		mapTaken  = first $ read . BS.unpack
		mapRemain = second $ BS.dropWhile Char.isSpace
		span' pred s
			| BS.null taken = Nothing
			| otherwise     = Just (taken, remain')
			where
				(taken, remain) = BS.span pred s
				remain' = BS.dropWhile Char.isSpace remain

readToListN :: Read a => Int -> ByteString -> [a]
readToListN n = take n . readToList

readIntToVector :: ByteString -> Vector Int
readIntToVector = Vector.unfoldr (fmap stripRemain . BS.readInt)
	where stripRemain = second $ BS.dropWhile Char.isSpace

readIntToUVector :: ByteString -> UVector.Vector Int
readIntToUVector = UVector.unfoldr (fmap stripRemain . BS.readInt)
	where stripRemain = second $ BS.dropWhile Char.isSpace

readIntToVectorN :: Int -> ByteString -> Vector.Vector Int
readIntToVectorN n = Vector.unfoldrN n (fmap stripRemain . BS.readInt)
	where stripRemain = second $ BS.dropWhile Char.isSpace

readIntToUVectorN :: Int -> ByteString -> UVector.Vector Int
readIntToUVectorN n = UVector.unfoldrN n (fmap stripRemain . BS.readInt)
	where stripRemain = second $ BS.dropWhile Char.isSpace


first :: (a -> a') -> (a, b) -> (a', b)
first f (a, b) = (f a, b)

second :: (b -> b') -> (a, b) -> (a, b')
second f (a, b) = (a, f b)

average :: Fractional a => [a] -> a
average xs = sum xs / (fromIntegral . length) xs

-- ----------
modIntDivisor :: Int
modIntDivisor = floor 1e9 + 7

extGcd :: Integral a => Int -> Int -> (Int, Int)
extGcd a 0 = (1, 0)
extGcd a b = (x, y)
	where
		(x', y') = extGcd b (a `mod` b)
		x = y'
		y = x' - a `div` b * y'

newtype ModInt = ModInt { unModInt :: Int } deriving Eq

modInt :: Int -> ModInt
modInt x = ModInt $ (x `mod` modIntDivisor + modIntDivisor) `mod` modIntDivisor

modIntMap2 :: (Int -> Int -> Int) -> ModInt -> ModInt -> ModInt
modIntMap2 f a b = modInt $ f (unModInt a) (unModInt b)

instance Show ModInt where
	show = show . unModInt

instance Read ModInt where
	readsPrec n s = map (first ModInt) $ readsPrec n s

instance Num ModInt where
	(+) = modIntMap2 (+)
	(-) = modIntMap2 (-)
	(*) = modIntMap2 (*)
	abs = id
	signum _ = 1
	fromInteger = modInt . fromInteger

instance Enum ModInt where
	toEnum = modInt
	fromEnum = unModInt

minv :: ModInt -> ModInt
minv a = modInt y
	where (_, y) = extGcd modIntDivisor $ unModInt a

mdiv :: ModInt -> ModInt -> ModInt
mdiv x y = x * minv y

mperm :: ModInt -> ModInt -> ModInt
mperm _ 0 = 1
mperm n r = product [(n - r + 1)..n]

mcomb :: ModInt -> ModInt -> ModInt
mcomb n r = mperm n r `mdiv` product [1..r]

combinations :: Int -> Int -> [ModInt]
combinations n k = zeros ++ body
	where
		zeros = replicate (k - 1) (ModInt 0)
		body  = scanl (comb k) 1 [0..(n - k - 1)]

comb :: Int -> ModInt -> Int -> ModInt
comb k acc n = acc * ((n' + k') `mdiv` (n' + 1))
	where
		n' = ModInt n
		k' = ModInt k
-- maximums :: Int -> Int -> [ModInt]

mmsums :: Int -> Int -> [Int] -> ModInt
mmsums n k as = maximums - minimums
	where
		as' = ModInt <$> List.sort as
		combs = combinations n k
		maximums = sum $ zipWith (*) as' combs
		minimums = sum $ zipWith (*) as' (reverse combs)

main :: IO ()
main = do
	[n, k] <- readToList <$> BS.getLine
	as <- readToListN n <$> BS.getLine
	print $ mmsums n k as


Measured memory_bytes: 23932