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 n) {
        // Return the largest value of any of A's nonempty subarrays.
        int i,j,max;
        int sum[]=new int[n];
        for(i=0;i<n;i++)
        {
            for(j=i+1;j<n-1;j++)
            {
                sum[i]=sum[i]+(A[i]*A[j+1]);
            }
        }
        max=sum[0];
        for(i=1;i<n;i++)
        {
            if(sum[i]>max)
            {
                max=sum[i];
            }
        }
        return max-1;
    }

    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,n);
        System.out.println(result);
        in.close();
    }
}