ai = lambda : map(int,raw_input().split())
n, k = ai()
c = ai()
bulbs = [1]*n
mincost = 10**20
for i in xrange(k+1):
  cost = c[i]
  j = i + 2*k+1
  while j < n:
    cost += c[j]
    j += 2*k+1
  mincost = min(mincost, cost)
  
print mincost