n,k=[int(x) for x in input().split()]
cost=[int(x) for x in input().split()]
cost2=[0]*n
bulb=[1]*n
for i in range(n):
    a=max(i-k,0)
    b=min(i+k,n-1)
    cost2[i]=2*cost[i]
    for c in range(a,b+1):
        cost2[i]-=cost[c] 
#print(cost2)
finalCost=0
while 1 in bulb:
    On_bulbs=[i for i,val in enumerate(bulb) if val==1]
    minc=cost2[On_bulbs[0]]
    mini=On_bulbs[0]
    for z in On_bulbs:
        if cost2[z]<minc:
            minc=cost2[z]
            mini=z
    finalCost+=cost[mini]
    a=max(mini-k,0)
    b=min(mini+k,n-1)
    for c in range(a,b+1):
        bulb[c]=0 

print(finalCost)