from heapq import heappush, heappop

N, K = map(int, input().split())
A = list(map(int, input().split()))

cum = [0]
for a in A:
    cum.append(cum[-1]+a)
cum.pop(0)

que = []
SUM = 0
ans = -(10**18)
for i in range(K-1, N):
    if i == K-1:
        for j in range(K-1):
            heappush(que, -cum[j])
            SUM += cum[j]
    else:
        if que and cum[i-1] < -que[0]:
            SUM -= -heappop(que)
            heappush(que, -cum[i-1])
    ans = max(ans, cum[i]*K-SUM)

print(ans)