/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
			Scanner sc=new Scanner(System.in);
		int n = sc.nextInt();
		int k = sc.nextInt();
		int[] a=new int[n];
		for(int i=0;i<n;i++){
		    a[i]=sc.nextInt();
		}
		int[] dp=new int[n];
		dp[0]=0;
 
		for(int i=0;i<n;i++){
		    int u=0;
		    for(int j=Math.max(u,i-k);j<=i-1;j++){
		        dp[i]=Math.min(dp[j],a[i]*a[j] + dp[j]);
		    }
		}
		System.out.println(dp[n-1]);
	}
}

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NSAzIAotMiA1IC0zIDYgLTE=

5 3 
-2 5 -3 6 -1