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 i=0,j=0;
        int prod=0,sum=0;
      for(i=0;i<A.length;i++)
      {
       for(j=i+1;j<A.length;j++)
       {
         prod=A[i]*A[j];
         sum=sum+prod;
       }
      }
        
        
        return sum;
    }

    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();
    }
}