from fileinput import input
import sys


data = input()

n, k = map(int, next(data).split())

bulbs = list(map(int, next(data).split()))
if k == 0:
    print(sum(bulbs))
    sys.exit(0)

cost = 0
for i in range(k, n, 2 * k + 1):
    cost += bulbs[i]
print(cost)