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();
        long[] test=new long[n];
        long min=Long.MAX_VALUE;
        for(int i=0;i<n;i++)
        {
            test[i]=in.nextLong();
            min=Math.min(min,test[i]);
        }    
        if(k>=n-1)
        {
            System.out.println(min);
            return;
        }
        long cost=Long.MAX_VALUE;
        for(int i=0;i<=k;i++)
        {
            int begin=i;
            long tCost=test[i];
            while(begin+k<n-1)
            {
                if(begin+k+k+1<n)
                {
                    begin+=k+k+1;
                    tCost+=test[begin];
                }    
                else
                {
                    tCost=Long.MAX_VALUE;
                    break;
                }    
            }
            cost=Math.min(cost,tCost);
        }
        System.out.println(cost);
    }
}