#include<bits/stdc++.h>
using namespace std;

typedef long long int ll;

#define pb push_back
#define mp make_pair
#define fi first
#define se second

void init()
{
    int n,k;
    cin >> n >> k ;
    int c[n+5],i;
    for(i=0;i<n;i++)
    cin >> c[i];
    if(k==0)
    {
        ll ans=0ll;
        for(i=0;i<n;i++)
        ans=(ll)ans+(ll)c[i];
        cout << ans ;
    }
    else
    {
        int idx=k;
        ll ans=0ll;
        ans+=((ll)c[idx]);
        while(idx<n)
        {
            idx+=((2*k)+1);
            ans+=((ll)c[idx]);
        }
            //if(idx>n-1)
            //ans+=((ll)c[n-1]);
            //  cout << ans ;
    }
}

int main()
{
    clock_t time1 = clock();
	#ifndef ONLINE_JUDGE
	//freopen("IN.in","r",stdin);
	#else
	//ONLINE_JUDGE
	#endif
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
    init();
    clock_t time2 = clock();
    cerr <<"Time: " <<(double)(time2-time1)/CLOCKS_PER_SEC * 100000 <<" seconds" <<endl;
	return 0;
}