from itertools import permutations
from bisect import bisect_left, bisect_right
import random
import math
from collections import deque
from collections import Counter
from collections import defaultdict
inf = 1 << 60


def sgn(x):
    if x > 0:
        return 1
    elif x == 0:
        return 0
    else:
        return -1


def popC(x):
    ans = 0
    while x != 0:
        ans += x % 2
        x //= 2
    return ans


def LI():
    return list(map(int, input().split()))


def II():
    return int(input())


def SI():
    return input()


n = II()
a = LI()
now = a[0]
ans = 0
for i in range(1, n):
    ans += a[i]*now
    now += a[i]
print(ans)