#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    int n,k;
    cin >> n >> k;
    vector<int> c(n);
    for (int i = 0; i < n; i++)
    {
        cin >> c[i];
    }
    vector<long> pmustnotoverlap(n);
    vector<long> pmayoverlap(n);
    for (int i = 0; i < n; i++)
    {
        if (i-k < 0)
        {
            pmustnotoverlap[i] = 0x7FFFFFFFFFFFFFFF;
        }
        else
        {
            pmustnotoverlap[i] = c[i-k];
            int r = i-2*k-1;
            if (r >= 0)
            {
                if (pmustnotoverlap[r] == 0x7FFFFFFFFFFFFFFF)
                    pmustnotoverlap[i] = pmustnotoverlap[r];
                else
                    pmustnotoverlap[i] += pmustnotoverlap[r];
            }
        }
        //cout << pmustnotoverlap[i] << endl;
    }
    for (int i = 0; i < n; i++)
    {
        long m = 0x7FFFFFFFFFFFFFFF;
        for (int j = i-k; j <= i+k; j++)
        {
            if (j < 0 || j >= n)
                continue;
            //turn on light j
            long cc = c[j];
            //turn on rest
            int r = j-k-1;
            if (r >= 0)
            {
                if (pmustnotoverlap[r] == 0x7FFFFFFFFFFFFFFF)
                    continue;
                cc += pmustnotoverlap[r];
            }
            m = min(m, cc);
        }
        pmayoverlap[i] = m;
        //cout << pmayoverlap[i] << endl;
    }
    cout << pmayoverlap[n-1];
}