# Enter your code here. Read input from STDIN. Print output to STDOUT
n,k=map(int,raw_input().strip().split())
a=map(int,raw_input().strip().split())
minsum=float("inf")
for i in range(0,k+1):
    sum1=sum(a[i::(2*k+1)])
    if sum1<minsum: minsum=sum1
print minsum