using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ProblemSolver
{
    class Solution
    {
        static char[] splitors = { ' ' };

        /* Input data here */
        static int n;
        static int k;
        static long[] c;        

        static void Input()
        {
            string[] str = Console.ReadLine().Split(splitors);
            n = int.Parse(str[0]);
            k = int.Parse(str[1]);
            str = Console.ReadLine().Split(splitors);
            c = new long[n];
            for (int i = 0; i < n; i++)
            {
                c[i] = long.Parse(str[i]);
            }
        }

        static void Search()
        {            
            long[] cost = new long[n];
            for (int i = 0; i < n; i++)
            {
                cost[i] = -1;
            }
            cost[n - 1] = 0;

            for (int i = n - 2; i >= 0; i--)
            {
                int next = i + k + 1;
                if (next < n)
                {
                    int j = Math.Min(next + k, n - 1);
                    // for (int j = i + 1; j <= next + k && j < n; j++)
                        if (cost[j] != -1)
                        {
                            if (cost[i] == -1)
                            {
                                cost[i] = cost[j] + c[next];
                            }
                            else
                            {
                                cost[i] = Math.Min(cost[i], cost[j] + c[next]);
                            }
                        }
                }
            }

            long ret = -1;
            for (int last = 0; last <= k && last < n; last++)
            {
                int j = Math.Min(last + k, n - 1);
                //for (int j = 0; j <= last + k && j < n; j++)
                    if (cost[j] != -1)
                    {
                        long val = cost[j] + c[last];
                        if (ret == -1)
                        {
                            ret = val;
                        }
                        else
                        {
                            ret = Math.Min(ret, val);
                        }
                    }
            }

            Console.WriteLine(ret);
        }

        static void Main(string[] args)
        {
            Input();
            Search();
        }
    }
}