#!/bin/python3

import sys
import itertools

def largestValue(A):
    pairs = itertools.combinations(A, 2)
    answer = 0
    for pair in pairs:
        answer += pair[0] * pair[1]
    l=0
    for i in range(answer):
        l+=(answer[i][0]*answer[i][1])
        
    return l       

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