using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;

class Solution {
    public class SwitchSum
    {
        public int Index;
        public long Cost;
        public bool StillOn;
    }
    
    static void Main(String[] args) {
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution */
        int[] nk = Array.ConvertAll(Console.ReadLine().Split(' '), Int32.Parse);
        int k = nk[1];
        int[] c = Array.ConvertAll(Console.ReadLine().Split(' '), Int32.Parse);
        
        List<SwitchSum> ss = new List<SwitchSum>();
        
        for (int i=0; i<nk[0]; i++)
        {
            int cSum = 0;
            for (int j=-k; j<=k; j++)
            {
                if (i+j < 0) continue;
                if (i+j >= nk[0]) continue;
                
                cSum += c[i+j];
            }
            ss.Add(new SwitchSum(){ Index = i, Cost = cSum, StillOn = true});
        }
        
        //Console.WriteLine(String.Join(" ", ss.Select(s => s.Cost).ToArray()));
        
        List<SwitchSum> sso = ss.OrderByDescending(s => s.Cost).ToList();
        
        long result = 0;
        for (int i=0; i<nk[0]; i++)
        {
            if (!sso[i].StillOn) continue;
            //Console.WriteLine(String.Join(" ", ss.Select(s => s.StillOn).ToArray()));            
            
            int ix = sso[i].Index;
            result += c[ix];
            
            //Console.WriteLine($"Switch {ix}");
            
            for (int j=-k; j<=k; j++)
            {
                if (ix+j < 0) continue;
                if (ix+j >= nk[0]) continue;
                
                ss[ix+j].StillOn = false;
            }            
            //Console.WriteLine(String.Join(" ", ss.Select(s => s.StillOn).ToArray()));
        }
        
        Console.WriteLine(result);
    }
}