import math
n, k = [int(i) for i in input().strip().split(" ")]
l = [int(i) for i in input().strip().split(" ")]

if k >= n:
    print(min(l))
elif k >= n//2:
    print(min(l[n//2], l[n//2 - 1], l[n//2 + 1]))
else:
    if n%(2*k+1) == 0:
        curr = 0
        pos = k 
        while pos < n:
            curr += l[pos]
            pos += 2*k + 1
        print(curr)
    else:
        mini = float('inf')
        for j in range(0,n%(2*k+1)):
            curr = 0
            pos = k - j
            while pos < n:
                curr += l[pos]
                pos += 2*k + 1
            if curr < mini:
                mini = curr
        print(mini)