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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
#define ld long double
#define ll long long
#define ull unsigned ll
#define pll pair<ll, ll>
#define itr(a) begin(a), end(a)
#define pb push_back
#define x first
#define y second
#define endl '\n'
#define FAST cin.tie(0); cout.tie(0); ios_base::sync_with_stdio(0);
#define FILES(namein, nameout) freopen(namein, "r", stdin); freopen(nameout, "w", stdout);
#define M 1000000007
#define N 105
#define INF 101111111111111
using namespace std;
ll a[N];
main(){
    FAST;
    ll n;
    cin >> n;
    for(ll i =0;i<n;i++) cin >> a[i];
    ll res = 0;
	for(ll i = 0;i<n;i++){
		for(ll j= i+1;j<n;j++) res += a[i]*a[j];
	}
	cout << res;
}