所求和问题

$（1+2+3{)}^2=1^2+2^2+3^2+2\ast (1\ast 2+1\ast 3+2\ast 3)$

n = input()
a = list(map(int,input().split()))
s1 = sum(a) ** 2 
s2 = 0
for i in a:
  s2 += i ** 2 
ans = (s1 - s2) // 2 
print(ans)