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 fastio cin.tie(NULL); cout.tie(NULL); ios_base::sync_with_stdio(0)
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vector<int>> matrix;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define all(c) c.begin(),c.end()
#define mp make_pair
#define pb push_back
#define f first
#define int ll
#define tcase int t; cin>> t; for( ; t; --t)
#define s second
#define endl '\n'
const int MAX = 1e6+1;
const int INF = 0x3f3f3f3f;
const double PI = acos(-1);
const double DEG = 180/PI;
const int MOD = 1e9+7;
int mod(int x, int m){ if(x >= 0) return x % m; if(-x < m) return m-(-x); return mod(x%m, m);}
int gcd (int a, int b) {while(b){a = mod(a, b); swap(a, b);} return a;}
int lcm (int a, int b) {return a / gcd(a, b) * b;}
int fexp(int base, int exp, int m){
	int total = 1;
	while(exp > 0){
		if(exp % 2 == 1){
			total = mod(((mod(total, m)) * (mod(base, m))), m);
			exp--;
		}
		base= mod((mod(base, m) * mod(base, m)), m);
		exp /= 2;
	}
	return mod(total, m);
}

int n;

int32_t main(){
	fastio;
	cin>> n;
	vi d(n);
	int ans = 0;
	for(int i = 0; i<n; ++i) cin>> d[i];
	for(int i = 0; i<n; ++i){
		for(int j = i+1; j<n; ++j) ans += d[i]*d[j];
	}
	cout<< ans<< endl;
	return 0;
}