using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
class Solution {

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

    static void Main(String[] args) {
        int n = Convert.ToInt32(Console.ReadLine());
        string[] A_temp = Console.ReadLine().Split(' ');
        int[] A = Array.ConvertAll(A_temp,Int32.Parse);
        long result = largestValue(A,n);
        Console.WriteLine(result);
    }
}