n,k = input().strip().split(' ')
n,k = [int(n),int(k)]
bulb = [int(t) for t in input().strip().split(' ')]
mini = 1000000007
for j in range(n):
    forward = j
    backward = j
    cost = bulb[j]
    flag = True
    flag1 = True
    while(flag or flag1):
        if(forward+k+1<n):
            forward+=(k+1)
        else:
            flag = False
        if(backward-k>0):
            backward-=(k+1)
        else:
            flag1 = False
        # print("forward backward",forward,backward)
        if(flag):
            cost += bulb[forward]
        if(flag1):
            cost += bulb[backward]
    if(cost<mini):
        mini = cost
print(mini)