n, k = map(int, raw_input().split())
ci = map(int, raw_input().split())


ci_en = []
for i in xrange(n):
	costi = ci[i]
	benefit = min(n, i + k) -  max(0, i - k) + 1
	ci_en.append([costi, benefit, i])

#ci_en = list(enumerate(ci))
# [(index, ci), ...]
#print list(ci_en)

ar = [True] * n

ci_en.sort(key=lambda x: float(x[1]) / x[0])

cost = 0
while True in ar:
	ci_cost, ci_ben, ci_idx = ci_en.pop()
	if ar[ci_idx] == False:
		continue
	else:
		cost += ci_cost
		for idx in xrange(max(0, ci_idx - k), min(n, ci_idx + k + 1)):
			ar[idx] = False
		# for bulb in xrange(n):
			# cos
			# costi = ci[i]
			# benefit = min(n - 1, i + k) -  max(0, i - k) + 1
			# ci_en.append([costi, benefit, i])
print cost