def profit(lamp, cost, k):
    p = []
    for i in range(len(lamp)):
        l = 0
        for j in range(max(0,i-k),min(i+k,len(lamp)-1)+1):
            if lamp[j] == 1:
                l += 1
        p.append(l/cost[i])
    return p
            


n,k = tuple(int(x) for x in input().strip().split())
lamp = [1 for i in range(n)]
cost = list(int(x) for x in input().strip().split())
total = 0

while [x for x in lamp if x ==1]:
    p = list(sorted(zip(profit(lamp, cost, k), range(len(lamp)))))
    s = p[-1][1]
    total += cost[s]
    for j in range(max(0,s-k),min(s+k,len(lamp)-1)+1):
        lamp[j] = 0
print(total)