B.py

n = int(input())
d = list(map(int, input().split()))
ans = 0
sum = sum(d)
for i in d:
    sum -= i
    ans += i*sum
print(ans)

#from itertools import combinations
# n = int(input())
# d = list(map(int, input().split()))
# ans = 0
# d = combinations(d, 2)
# for i, j in d:
#     ans += i*j
# print(ans)