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.
        return 0;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int[] A = new int[n];
        long mul=0;
        for(int A_i = 0; A_i < n; A_i++){
            A[A_i] = in.nextInt();
        }
        for(int i=0;i<n-1;i++)
        {
            for(int j=i+1;j<n;j++)
            {
                mul=mul+A[i]*A[j];
            }
            
        }
        long result = largestValue(A);
        System.out.println(mul);
        in.close();
    }
}