# Enter your code here. Read input from STDIN. Print output to STDOUT
n,k=map(int,raw_input().split())
arrC=map(int,raw_input().split())
best=10**15
for i in xrange(min(n,k+1)):
  ids=range(i,n,2*k+1)
  if ids[-1]+k>=n-1:
    act=sum([arrC[j] for j in ids])
    best=min(best,act)  
print best