n, k = map(int, raw_input().split())
A = map(int, raw_input().split())
ans = 10 ** 20
for i in xrange(min(k + 1, n)):
	_ans = 0
	for j in xrange(i, n, 2 * k + 1):
		_ans += A[j]
	if j + k >= n - 1 or i + k >= n - 1:
		ans = min(ans, _ans)
print ans