def cost(C, n, k, offset):
    bulbs = k*2+1
    return sum(C[offset::bulbs])

def solve(C, n, k):
    return min([cost(C, n, k, offset) for offset in range(k*2+1)])

n, k = list(map(int, input().strip().split(" ")))
C = list(map(int, input().strip().split(" ")))
result = solve(C, n, k)
print(result)