include [HTML_REMOVED]
int main()
{
int n, array[200000+10] = { 0 };
long long sum = 0;
scanf(“%d”, &n);
for (int i = 1; i <= n; i++)
scanf(“%d”, &array[i]);
long long s[200000+10];
/*for (int i = 0; i < n - 1; i++)
for (int j = i + 1; j < n; j++)
{
ans += array[i] * array[j];
}*/
s[0] = 0;
for (int i = 1; i <= n; i++)
s[i] = s[i - 1] + array[i];
for (int i = 1; i <= n; i++)
sum += array[i] * (s[n] - s[i]);
printf("%lld", sum);
return 0;
}