Problem:
Score : 200 points

Problem Statement

It's now the season of TAKOYAKI FESTIVAL!

This year, N takoyaki (a ball-shaped food with a piece of octopus inside) will be served. The deliciousness of the i-th takoyaki is d_i.

As is commonly known, when you eat two takoyaki of deliciousness x and y together, you restore x \times y health points.

There are \frac{N \times (N - 1)}{2} ways to choose two from the N takoyaki served in the festival. For each of these choices, find the health points restored from eating the two takoyaki, then compute the sum of these \frac{N \times (N - 1)}{2} values.

Constraints

All values in input are integers.

2 \leq N \leq 50

0 \leq d_i \leq 100

Input

Input is given from Standard Input in the following format:

N
d_1 d_2 ... d_N

Output

Print the sum of the health points restored from eating two takoyaki over all possible choices of two takoyaki from the N takoyaki served.

Sample Input 1

3
3 1 2

Sample Output 1

11

There are three possible choices:

Eat the first and second takoyaki. You will restore 3 health points.

Eat the second and third takoyaki. You will restore 2 health points.

Eat the first and third takoyaki. You will restore 6 health points.

The sum of these values is 11.

Sample Input 2

7
5 0 7 8 3 3 2

Sample Output 2

312

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace::std;

#define all(x) (x).begin(), (x).end()

typedef long long ll;
typedef array<int, 3> tri;
typedef long double ld;

template <class T> istream& operator>>(istream& I, vector<T>& v) {for (T &e: v) I >> e; return I;}
template <class T> ostream& operator<<(ostream &O, const vector<T>& v) {for (const T &e: v) O << e << ' '; return O;}



void _main() {
        int n; cin >> n;
        vector<int> d(n); cin >> d;
        int ans = 0;
        for (int i = 0; i < n; i++)
                for (int j = i + 1; j < n; j++)
                        ans += d[i] * d[j];

        cout << ans;
}



signed main() {
        ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
//        freopen("input.txt", "r", stdin);
        int _t = 1;
//        cin >> _t;
        while (_t--) _main();
        return 0;
}