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

public class Solution
{

    static long largestValue(long[] A)
    {
        long product = 0;
        int n = A.length;
        for (int i = 0; i < n; i++)
            for (int j = i+1; j < n; j++)
                product = product + A[i]*A[j];
        return product;
    }

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