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

public class Solution {

    static long largestValue(int[] A) {
        // Return the largest value of any of A's nonempty subarrays.
        int n = A.length;
        if (n == 0) return 0;
        if (n == 1) return A[0];
        long[][] sum = new long[n][n];
        long[][] ss = new long[n][n];
        
        long max = 0;
        
        for (int i = 1; i < n; i++) {
            for (int j = 0; j < n - i; j++) {
                int x = j;
                int y = j + i;
                
                if (x == y - 1) {
                    sum[x][y] = A[x] + A[y];
                    ss[x][y] = A[x] * A[x] + A[y] * A[y];
                } else {
                    sum[x][y] = sum[x][y - 1] + A[y];
                    ss[x][y] = ss[x][y - 1] + A[y] * A[y];
                }
                
                max = Math.max(max, (sum[x][y] * sum[x][y] - ss[x][y]) / 2);
            }
        }
        
        return max;
    }

    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();
    }
}