简单前缀和
#include <bits/stdc++.h>
using namespace std;
long long n,a[200010],s[200010],ans,x;
int main() {
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> a[i];
s[i] = s[i - 1] + a[i];
}
for (int i = 1; i <= n; i++) {
ans += a[i] * (s[n] - s[i]);
}
cout << ans;
}