import sys
input = sys.stdin.readline
from collections import *

N = int(input())
d = list(map(int, input().split()))
ans = 0

for i in range(N):
    for j in range(i+1, N):
        ans += d[i]*d[j]

print(ans) import sys sys input = sys.stdin.readline input sys.stdin.readline sys.stdin sys stdin readline from collections import * N = int(input()) N int(input()) int input() input d = list(map(int, input().split())) d list(map(int, input().split())) list map(int, input().split()) map int input().split() ().split () input split ans = 0 ans 0 for i in range(N):
    for j in range(i+1, N):
        ans += d[i]*d[j] i range(N) range N for j in range(i+1, N):
        ans += d[i]*d[j] j range(i+1, N) range i+1 i 1 N ans += d[i]*d[j] ans d[i]*d[j] d[i] d i d[j] d j print(ans) print ans ans += d[i]*d[j] d[i]*d[j] ans d = list(map(int, input().split())) list(map(int, input().split())) d N = int(input()) int(input()) N ans = 0 0 ans input = sys.stdin.readline sys.stdin.readline input
import sys
input = sys.stdin.readline
from collections import *

N = int(input())
d = list(map(int, input().split()))
ans = 0

for i in range(N):
    for j in range(i+1, N):
        ans += d[i]*d[j]

print(ans)