import java.util.*;
import java.io.*;
class FastReader
{
BufferedReader br;
StringTokenizer st;

public FastReader()
{
	br = new BufferedReader(new
			 InputStreamReader(System.in));
}

String next()
{
	while (st == null || !st.hasMoreElements())
	{
		try
		{
			st = new StringTokenizer(br.readLine());
		}
		catch (IOException  e)
		{
			e.printStackTrace();
		}
	}
	return st.nextToken();
}

int nextInt()
{
	return Integer.parseInt(next());
}

long nextLong()
{
	return Long.parseLong(next());
}

double nextDouble()
{
	return Double.parseDouble(next());
}

String nextLine()
{
	String str = "";
	try
	{
		str = br.readLine();
	}
	catch (IOException e)
	{
		e.printStackTrace();
	}
	return str;
}
}
class Solution 
{
	FastReader sc;
	int n,k;
	long cost[];
	int arr[];
	public Solution()
	{
		sc=new FastReader();
		n=sc.nextInt();
		k=sc.nextInt();
		cost=new long[n];
		arr=new int[n];
		long mincost=Long.MAX_VALUE;
		Arrays.fill(arr,1);
		for(int i=0;i<n;i++)
		{
			cost[i]=sc.nextInt();
		}
		long ct=0;
		for(int i=0;i<=k;i++)
		{
			ct=0;
			Arrays.fill(arr,1);
			for(int j=i;j<n;j=j+2*k+1)
			{
				ct=ct+cost[j];
				for(int x=Math.max(j-k,0);x<=Math.min(j+k,n-1);x++)
				{
					arr[x]=0;
				}
			}
			int flag=0;
			for(int j=0;j<n;j++)
			{
				if(arr[j]==1)
				{
					flag=1;
					break;
				}
			}
			if(flag==0)
			{
				if(ct<mincost)
				{
					mincost=ct;
				}
			}
		}
		System.out.println(mincost);

	}
	public static void main(String[] args) 
	{
		new Solution();
	}
}