# Enter your code here. Read input from STDIN. Print output to STDOUT

n,k = map(int,raw_input().split(' '))
c = k*2+1
costs = map(int,raw_input().split(' '))

naiveBest = [x+1 for x in range(0,n-1,c)]

curSum = 0
for i in naiveBest:
    curSum += costs[i]
    
print curSum