#!/bin/python

import sys

def largestValue(ar):
    s=0
    for i in range(0,len(ar)):
        for j in range(i+1,len(ar)):
            s=s+ar[i]*ar[j]
    return s
    # 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)
    print result