import sys
input=sys.stdin.readline
n=int(input())
a=list(map(int,input().split()))
ans=0
s=[0]*(n+5)
for i in range(1,n+1):
  s[i]=s[i-1]+a[i-1]
for i in range(0,n-1):
  ans+=a[i]*(s[n]-s[i+1])
print(ans)