import bisect
import copy
import decimal
import fractions
import heapq
import itertools
import math
import random
import sys
import time
from collections import Counter,deque,defaultdict
from functools import lru_cache,reduce
from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max
def _heappush_max(heap,item):
    heap.append(item)
    heapq._siftdown_max(heap, 0, len(heap)-1)
def _heappushpop_max(heap, item):
    if heap and item < heap[0]:
        item, heap[0] = heap[0], item
        heapq._siftup_max(heap, 0)
    return item
from math import gcd as GCD
read=sys.stdin.read
readline=sys.stdin.readline
readlines=sys.stdin.readlines
write=sys.stdout.write

class Slope_Trick:
    def __init__(self,L=False,R=False,median=False,label=False,f=None,f_inve=None,e=None):
        self.queueL=[]
        self.queueR=[]
        self.L=L
        self.R=R
        self.median=median
        if self.median:
            self.median_value=None
        self.label=label
        if self.label:
            self.f=f
            self.f_inve=f_inve
            self.e=e
            self.labelL=self.e
            self.labelR=self.e

    def heappush(self,x):
        heappush(self.queueR,x)
        if self.label:
            self.labelR=self.f(self.labelR,x)

    def heappop(self):
        x=heappop(self.queueR)
        if self.label:
            self.labelR=self.f_inve(self.labelR,x)
        return x

    def heappushpop(self,x):
        y=heappushpop(self.queueR,x)
        if self.label:
            self.labelR=self.f(self.labelR,x)
            self.labelR=self.f_inve(self.labelR,y)
        return y

    def _heappush_max(self,x):
        _heappush_max(self.queueL,x)
        if self.label:
            self.labelL=self.f(self.labelL,x)

    def _heappop_max(self):
        x=_heappop_max(self.queueL)
        if self.label:
            self.labelL=self.f_inve(self.labelL,x)
        return x

    def _heappushpop_max(self,x):
        y=_heappushpop_max(self.queueL,x)
        if self.label:
            self.labelL=self.f(self.labelL,x)
            self.labelL=self.f_inve(self.labelL,y)
        return y

    def push_Left(self,x):
        self._heappush_max(self.heappushpop(x))

    def push_Right(self,x):
        self.heappush(self._heappushpop_max(x))

    def pop_Left(self):
        if self.queueL:
            retu=self._heappop_max()
        else:
            retu=None
        return retu

    def pop_Right(self):
        if self.queueR:
            retu=self.heappop()
        else:
            retu=None
        return retu

    def push(self,x):
        if self.L:
            if len(self.queueL)<self.L:
                self.push_Left(x)
            else:
                self.push_Right(x)
        if self.R:
            if len(self.queueR)<self.R:
                self.push_Right(x)
            else:
                self.push_Left(x)
        if self.median:
            if self.median_value==None:
                if self.queueL and x<self.queueL[0]:
                    self.median_value=self._heappushpop_max(x)
                elif self.queueR and self.queueR[0]<x:
                    self.median_value=self.heappushpop(x)
                else:
                    self.median_value=x
            else:
                if self.median_value<=x:
                    self._heappush_max(self.median_value)
                    self.heappush(x)
                else:
                    self._heappush_max(x)
                    self.heappush(self.median_value)
                self.median_value=None

    def pop(self):
        if self.L:
            if len(self.queueL)==self.L:
                retu=self._heappop_max()
                if self.queueR:
                    self._heappush_max(self.heappop())
            else:
                retu=None
        if self.R:
            if len(self.queueR)==self.R:
                retu=self.heappop()
                if self.queueL:
                    self.heappush(self._heappop_max())
            else:
                retu=None
        if self.median:
            if self.median_value==None:
                retu=(self.Pop_Left(),self.Pop_Right())
            else:
                retu=(self.median_value,None)
                self.median_value=None
        return retu

    def top(self):
        if self.L:
            if len(self.queueL)==self.L:
                retu=self.queueL[0]
            else:
                retu=None
        if self.R:
            if len(self.queueR)==self.R:
                retu=self.queueR[0]
            else:
                retu=None
        if self.median:
            if self.median_value==None:
                if self.queueL:
                    retu=(self.queueL[0],self.queueR[0])
                else:
                    retu=(None,None)
            else:
                retu=(self.median_value,None)
        return retu

    def Label_Left(self):
        return self.labelL

    def Label_Right(self):
        return self.labelR

    def __len__(self):
        retu=len(self.queueL)+len(self.queueR)
        if self.median and self.median_value!=None:
            retu+=1
        return retu

    def __str__(self):
        if self.median:
            if self.median_value==None:
                return "["+", ".join(map(str,sorted(self.queueL)))+"]+["+", ".join(map(str,sorted(self.queueR)))+"]"
            else:
                return "["+", ".join(map(str,sorted(self.queueL)))+"]+"+str(self.median_value)+"+["+", ".join(map(str,sorted(self.queueR)))+"]"
        else:
            return "["+", ".join(map(str,sorted(self.queueL)))+"]+["+", ".join(map(str,sorted(self.queueR)))+"]"

N,K=map(int,readline().split())
A=list(map(int,readline().split()))
ST=Slope_Trick(R=K-1,label=True,f=lambda x,y:x+y,f_inve=lambda x,y:x-y,e=0)
ans=0
for i in range(N-1,-1,-1):
    if i%2:
        ans=max(ans,ST.Label_Right()+A[i])
    ST.push(A[i])
print(ans)