n, k = map(int, input().strip().split())
costs = list(map(int, input().strip().split()))
r, m = divmod((n-1),(2*k+1))
r += 1
q = k+1-abs(m-k)
nodes = []
for i in range(m):
    if (m-k > 0):
        nodes.append(k-i)
    else:
        nodes.append(i)
best = 0
for i in range(r):
    best += costs[i+nodes[0]]
for j in nodes[1:]:
    total = 0
    for i in range(r):
        total += costs[i+j]
    if (total < best):
        best = total
print(best)