#include<bits/stdc++.h>
#define rep(i,x,y) for(i=x;i<y;i++)
#define pb(f) push_back(f)
#define ll long long
#define mp make_pair
#define fi first
#define se second
#define mod 1000000007
using namespace std;

//#include<windows.h>
//FILE *fin = freopen("33.in","r",stdin);  FILE *fout = freopen("out.txt","w",stdout);
using namespace std;
typedef pair<ll, ll> pii;
typedef vector<ll > vi;
typedef vector< pii > vpii;

const int N=2e5+5;
map<pii ,ll>M;
ll dp[N],c[N];
int main()
{
	ll n,k,i,j,l;
	cin>>n>>k;
	rep(i,1,n+1)
	{
		cin>>c[i];
		dp[i]=1e17;
	}
	dp[0]=0;
	ll ans=1e17;
	rep(i,1,k+2)
	{
		ll cost=0;
		for(j=i;j<=n;)
		{
			cost+=c[j];
			j=j+2*k+1;
		}
		ans=min(ans,cost);
	}
	cout<<ans<<endl;
}