{-# OPTIONS_GHC -O2 #-}
{-# LANGUAGE BangPatterns          #-}
{-# LANGUAGE CPP                   #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE RecordWildCards       #-}
{-# LANGUAGE TupleSections         #-}
{-# LANGUAGE TypeFamilies          #-}

import           Control.Applicative
import           Control.Monad
import           Control.Monad.Primitive
import           Control.Monad.ST
import           Control.Monad.State.Strict
import           Data.Bool
import qualified Data.ByteString.Char8       as B
import qualified Data.ByteString.Unsafe      as B
import           Data.Char
import qualified Data.Foldable               as F
import           Data.Function
import qualified Data.IntMap.Strict          as IM
import qualified Data.IntSet                 as IS
import           Data.List
import qualified Data.Map.Strict             as M
import           Data.Monoid
import           Data.Ord
import           Data.Primitive.MutVar
import qualified Data.Set                    as S
import           Data.Tuple
import qualified Data.Vector                 as V
import qualified Data.Vector.Generic         as G
import qualified Data.Vector.Generic.Mutable as GM
import qualified Data.Vector.Unboxed         as U
import qualified Data.Vector.Unboxed.Mutable as UM
import           Debug.Trace
import           Foreign                     hiding (void)
import           Unsafe.Coerce

main :: IO ()
main = do
    [n, m] <- map read.words <$> getLine :: IO [Int]
    xs <- U.unfoldrN m (B.readInt.B.dropWhile isSpace) <$> B.getLine
    print $ solve n m xs

solve :: Int -> Int -> U.Vector Int -> Int
solve n m xs = U.sum
    . U.take (m - n)
    . radixSort32
    $ U.zipWith (-) (U.tail ys) ys
  where
    ys = radixSort32 $ U.map (+ 100000) xs

radixSort32 :: U.Vector Int -> U.Vector Int
radixSort32 v = F.foldl' step v [0, 16]
  where
    mask k x = fromIntegral $ unsafeShiftR x k .&. 0xffff
    step v k = U.create $ do
        pref <- U.unsafeThaw
            . U.prescanl' (+) 0
            . U.unsafeAccumulate (+) (U.replicate 0x10000 0)
            $ U.map ((,1) . mask k) v
        res <- UM.unsafeNew $ U.length v
        U.forM_ v $ \x -> do
            let !masked = mask k x
            i <- UM.unsafeRead pref masked
            UM.unsafeWrite pref masked $ i + 1
            UM.unsafeWrite res i x
        return res