n, k = map(int, raw_input().split())
c = map(int, raw_input().split())
l = 2 * k + 1
res = 10 ** 16
for i in range(k + 1):
    j = i
    a = 0
    while j < n:
        a += c[j]
        j += l
    if (j - l + k + 1) >= n:
        res = min(res, a)
print res