Sum of all pairwise products

You are given $n$ integers $x_1, x_2, \dots, x_n$. For every way of choosing two indices with $a < b$, add up $x_a x_b$.

$\sum_{1 \le a < b \le n} x_a x_b$

The first line contains the integer $n$. The integers $x_1, x_2, \dots, x_n$ follow, separated by spaces or line breaks.

$1 \le n \le 100000$, $|x_i| \le 100$

Print the sum of the products over all pairs on one line. The absolute value of the answer is at most $5 \times 10^{13}$.