#!/bin/python

import sys

def largestValue(A,n):
    sum=0
    for i in range(n):
        for j in range(i+1,n):
            sum+=A[i]*A[j]
    return sum
    # Return the largest value of any of A's nonempty subarrays.

if __name__ == "__main__":
    n = int(raw_input().strip())
    A = map(int, raw_input().strip().split(' '))
    result = largestValue(A,n)
    print result