#!/bin/python

import sys

def largestValue(A):
    # Return the largest value of any of A's nonempty subarrays.
    a=sum(A)
    a=a*a
    s=0
    for i in A:
        
        s= s + i **2
    return (a-s)/2
if __name__ == "__main__":
    n = int(raw_input().strip())
    A = map(int, raw_input().strip().split(' '))
    result = largestValue(A)
    print result