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

ans = 0

for i in range(N):
  for j in range(N):
    if i != j:
      ans += d[i]*d[j]
      
print(ans//2) 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(N):
    if i != j:
      ans += d[i]*d[j]
       i range(N) range N for j in range(N):
    if i != j:
      ans += d[i]*d[j]
       j range(N) range N if i != j:
      ans += d[i]*d[j]
       i != j i j ans += d[i]*d[j] ans d[i]*d[j] d[i] d i d[j] d j print(ans//2) print ans//2 ans 2 ans = 0 0 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
N = int(input())
d = list(map(int, input().split()))

ans = 0

for i in range(N):
  for j in range(N):
    if i != j:
      ans += d[i]*d[j]
      
print(ans//2)
