import java.util.Scanner;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        sc.nextLine();
        String[] split = sc.nextLine().split(" ");
        int[] c = new int[n];
        for (int i = 0; i < n; i++) {
            c[i] = Integer.parseInt(split[i]);
        }

        long minPrice = Long.MAX_VALUE;

        for (int ch = 0; ch <= k && ch < n; ch++) {
            int rem = n - k - ch - 1;
            rem = rem % (2 * k + 1);
            if (rem > 0 && rem <= k) {
                continue;
            }
            long price = 0;

            for (int i = ch; i < n; i += 2 * k + 1) {
                price += c[i];
            }

            if (price < minPrice) {
                minPrice = price;
            }
        }
        System.out.println(minPrice);
    }
}