import java.util.Scanner;
import java.util.function.Consumer;


public class Solution {

	static int n,k;
	static Long[] cost;
	static Long[][] mem;
	
	public static void main(String[] args) {
		 n = getInt();
		 k = getInt();
		 cost = getArrayLong(n);
		 long ans = Integer.MAX_VALUE;
		 for(int i=0; i<2*k+1; i++){
			 long curr = 0;
			 int j = i;
			 for(;j<n; j+=(2*k+1)){
				 curr+=cost[j];
			 }
			 if(n<=j-k){
				 ans=Math.min(ans, curr);
			 }
		 }
		 System.out.println(ans);
	}

	
	
	
	
	
	static Scanner s = new Scanner(System.in);
	static public int getInt(){ return s.nextInt(); }
	static public long getLong(){ return s.nextLong(); }
	static public String getString(){ return s.next();}
	static public Integer[] getArrayInt(int size){
		Integer[] ans = new Integer[size];
		for(int i=0; i<size; i++)
			ans[i]=getInt();
		return ans;
	}
	static public Long[] getArrayLong(int size){
		Long[] ans = new Long[size];
		for(int i=0; i<size; i++)
			ans[i]=getLong();
		return ans;
	}
	static public void forn(String str, Consumer<Character> c){	
		for(int i=0; i<str.length(); i++){
			c.accept(str.charAt(i));
		}
	}
	static public <T> void forn(T[] arr, Consumer<T> c){
		for(int i=0; i<arr.length; i++){
			c.accept(arr[i]);
		}
	}
	static public class Pair<T extends Comparable<T>, S extends Comparable<S>> implements Comparable<Pair<T, S>>{
		
		T first;
		S second;
		
		public Pair(T first, S second){
			this.first = first;
			this.second = second;
		}
		
		@Override
		public int compareTo(Pair<T, S> other) {
			int cmp = first.compareTo(other.first);
			if(cmp==0)
				return second.compareTo(other.second);
			return cmp;
		}
		
		@Override
		public int hashCode() {
			final int prime = 31;
			int result = 1;
			result = prime * result + ((first == null) ? 0 : first.hashCode());
			result = prime * result + ((second == null) ? 0 : second.hashCode());
			return result;
		}
		
		@Override
		public boolean equals(Object obj) {
			if (this == obj)
				return true;
			if (obj == null)
				return false;
			if (getClass() != obj.getClass())
				return false;
			Pair other = (Pair) obj;
			if (first == null) {
				if (other.first != null)
					return false;
			} else if (!first.equals(other.first))
				return false;
			if (second == null) {
				if (other.second != null)
					return false;
			} else if (!second.equals(other.second))
				return false;
			return true;
		}
	}
}