import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.util.Arrays;
import java.util.ArrayList;

public class Pro3 {
	public static void main(String[] args) throws IOException{ 
    	BufferedReader scan = new BufferedReader(new InputStreamReader(System.in));
    	String s[] = scan.readLine().split(" ");
    	String costs[] = scan.readLine().split(" ");
    	int n = Integer.parseInt(s[0]);
    	int k = Integer.parseInt(s[1]);
    	int set = k+k+1;
    	int ans = 0;
    	for(int i=0;i<n;i+=set){
    		int min = 9999999;
    		for(int j=0;j<set && i+j<n;j++){
    			if(Integer.parseInt(costs[i+j])<min)
    				min = Integer.parseInt(costs[i+j]);
    		}
    		ans = ans+min;
    	}
    	System.out.println(ans);
	}
}