import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    static long largestValue(int[] A) {
        int sum=0,sum2=0,d=A.length;
        for (int k=0;k<d;k++)
        {
            sum=0;
        for (int i=k;i<d;i++)
        {
            for (int j=i+1;j<d;j++)
            {
                sum=sum+A[i]*A[j];                     
                }                    
        }
       if (sum2<sum)
               
       {
           sum2=sum;
           sum=0;
       }
         d--;
            if (d<0)
                break;
    }
        return(sum2);
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int[] A = new int[n];
        for(int A_i = 0; A_i < n; A_i++){
            A[A_i] = in.nextInt();
        }
        long result = largestValue(A);
        System.out.println(result);
        in.close();
    }
}