#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <ctype.h>
#include <deque>
#include <queue>
#include <cstring>
#include <set>
#include <list>
#include <map>
#include <random>
#include <unordered_map>
#include <stdio.h>

using namespace std;

typedef long long ll;
typedef std::vector<int> vi;
typedef std::vector<bool> vb;
typedef std::vector<string> vs;
typedef std::vector<double> vd;
typedef std::vector<long long> vll;
typedef std::vector<std::vector<int> > vvi;
typedef vector<vvi> vvvi;
typedef vector<vll> vvll;
typedef std::vector<std::pair<int, int> > vpi;
typedef vector<vpi> vvpi;
typedef std::pair<int, int> pi;
typedef std::pair<ll, ll> pll;
typedef std::vector<pll> vpll;

const long long mod = 1000000007;

#define all(c) (c).begin(),(c).end()
#define sz(c) (int)(c).size()
#define forn(i, a, b) for(int i = a; i < b; i++)

#define pb push_back
#define mp make_pair

int main()
{
//#ifndef ONLINE_JUDGE
//    freopen("input.txt", "rt", stdin);
//    freopen("output.txt", "wt", stdout);
//#endif
    int n,k;
    scanf("%d %d", &n, &k);
    vll a(n);
    forn(i,0,n) {
        scanf("%lld", &a[i]);
    }
    ll best = 1e18;
    forn(i,0,min(n,k+1)) {
        ll ans =0;
        int cur = i;
        while(cur < n) {
            ans+=a[cur];
            cur+=2*k+1;
        }
        
        if(cur>=n+k) best=min(ans,best);
    }
    cout<<best;
}