n, k = map(int, raw_input().split())
c = map(int, raw_input().split())
s = (k << 1) + 1
answer = None
for i in xrange(k + 1):
    t = 0
    for j in xrange(i, n, s):
        t += c[j]
        if j + s >= n and j + k >= n - 1:
            if answer is None or t < answer:
                answer = t
print answer