from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
input = sys.stdin.readline

N = int(input())
A = list(map(int,input().split()))
C = Counter(A)
ans = 0
for k,v in C.items():
    for k2,v2 in C.items():
        ans += v*v2*pow(k,k2)
print(ans)