def main():
  n=int(input())
  d= list(map(int,input().split()))
  ans=0
  for i in range(0,n):
    for j in range(i+1,n):
      ans+=d[i]*d[j]
  print(ans)

main() def main():
  n=int(input())
  d= list(map(int,input().split()))
  ans=0
  for i in range(0,n):
    for j in range(i+1,n):
      ans+=d[i]*d[j]
  print(ans) main 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(0,n):
    for j in range(i+1,n):
      ans+=d[i]*d[j]
   i range(0,n) range 0 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 main() main def main():
  n=int(input())
  d= list(map(int,input().split()))
  ans=0
  for i in range(0,n):
    for j in range(i+1,n):
      ans+=d[i]*d[j]
  print(ans) def main():
  n=int(input())
  d= list(map(int,input().split()))
  ans=0
  for i in range(0,n):
    for j in range(i+1,n):
      ans+=d[i]*d[j]
  print(ans) main
def main():
  n=int(input())
  d= list(map(int,input().split()))
  ans=0
  for i in range(0,n):
    for j in range(i+1,n):
      ans+=d[i]*d[j]
  print(ans)

main()