#include <iostream>
#include <string>
#include <stdio.h>
#include <fstream>
#include <algorithm>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <cmath>
#include <cassert>
using namespace std;
typedef long long ll;
typedef long double ld;
#define PI (ld)3.14159265358979324
#define mp make_pair 
#define pb push_back
#define ft first
// #define sd second
#define sz(a) (ll)a.size()
#define pii pair<int,int>
#define pll pair<ll,ll>
#define loop(i, n) for(int (i) = 0; (i) < (n) ; ++ (i))
#define forn(i, l, r) for(int (i) = (l); (i) < (r) ; ++ (i))
#define vec vector 
#define str string
/*******************************************************/

int n,k;
vec<ll> c;

ll check(int k0) {
    ll ans = c[k0];
    int p = k0 + 2ll * k + 1;
    int rb = k0 + k;
    while(p < n) {
        ans += c[p];
        rb = p + k;
        p += 2ll * k + 1;
    }

    if(rb >= n - 1) return ans;
    return 1e15;
}

void solve() {
    cin >> n >> k;
    c.resize(n);
    loop(i,n) cin >> c[i];
    if(k >= n - 1) {
        ll ans = c[0];
        loop(i,n) ans = min(ans,c[i]);
        cout <<ans;
        return;
    }
    ll ans = 1e15;
    loop(i,k + 1) {
        ans = min(ans,check(i));
    }
    cout <<ans;
}


int main()
{
    // freopen("in.txt","r", stdin);
    ios_base::sync_with_stdio(false);   
    // cout << fixed;
    // cout.precision(3);

    solve();
    cout <<endl;
    return 0;
}