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 in=new Scanner(System.in);
        int n=in.nextInt();
        int k=in.nextInt();
        int ar[]=new int[n];
        int i,l;
        long sum=0,s=0;
        for(i=0;i<n;i++)
            {ar[i]=in.nextInt();
            sum=sum+ar[i];}
       if(k<n)
           {
           s=s+ar[k];
           l=k*2+1;
        for(i=l;i<n;i=i+l)
            {
            s=s+ar[i];
        }
           System.out.println(s);
       }
        else{System.out.println(sum);}
    }
}