#include<bits/stdc++.h> int main() { int n; cin >> n; vector<int> values(n); long long sum = 0; for (int i = 0; i < n; ++i) { cin >> values[i]; sum += values[i]; } long long Ans = 0; long long tmp = (n - 1) * (n - 2) / 2; for (int i = 0; i < n; ++i) { Ans += values[i] * tmp; } cout << Ans << endl; return 0; }