import java.io.*;
import java.util.*;

public class Solution {

    public static void main(String[] args) {
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        long cost[] = new long[n];
        for (int i = 0; i < n; i++) 
        {
            cost[i] = sc.nextLong();
        }
        int full = 2*k + 1;
        int remainder = (n % full) + 2*k + 1;
        long result = Long.MAX_VALUE;
        for (int j = k; j + k <= remainder; j++) 
        {
            long counter = 0;
            int c = j;
            while (c < cost.length) 
            {
                counter += cost[c];
                c += full;
            }
            if (counter < result) {result = counter;}
        }
        System.out.println(result);
    }
}