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

public class Solution {

    private static Scanner in = new Scanner(System.in);
    
    public static void main(String[] args) {
        int n = in.nextInt();
        int k = in.nextInt();
        long totalCost = 0;
        long[] c = new long[n];
        for (int i = 0; i < n; i++) {
            c[i] = in.nextLong();
        }
        int range = 1 + (2 * k);
        int mid = 1 + k;
        int mainTimes = n / range;
        for (int t = 0; t < mainTimes; t++) {
            int index = mid * (1 + t) + k * t;
            totalCost += c[index];
        }
        System.out.println(totalCost);
        in.close();
    }
}