import java.util.*;
import java.math.*;
public class Hack42_4 {
	
 static Scanner sc = new Scanner(System.in);
 static int mod = 1000000007;	
 
 public static void main(String[] args)
 {
	 int n = ni();
	 int k = ni();
	 int minCost = Integer.MAX_VALUE;
	 int[] l = ia(n);
	 for(int i=0;i<=k+1;i++)
	 {
		 int ans = 0;
		 int p = i;
		 while(p<n)
		 {
			 ans+=l[p];
			 p = p + 2*k + 1;
		 }
		 minCost = Math.min(minCost,ans);
	 }
	 System.out.println(minCost);
 }
 
 static long[] la(int x)
 {
	 long[] a = new long[x];
	 for(int i=0;i<x;i++)
		 a[i]=sc.nextLong();
	 return a;
 }
 
 static int[] ia(int x)
 {
	 int[] a = new int[x];
	 for(int i=0;i<x;i++)
		 a[i]=sc.nextInt();
	 return a;
 }
 
 static char[] ca()
 { return sc.next().toCharArray(); }
 
 static int ni()
 { return sc.nextInt();}
 
 static long nl()
 { return sc.nextLong();}
 
 static double nd()
 { return sc.nextDouble();}
 
}