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)

/**
 * Training to the municipal stage of ROI
**/ 
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define forn for(int i=0;i<n;i++)
#define form for(int j=0;j<m;j++)
#define pb push_back
#define mp make_pair
#define s second
#define f first
#define kek cout<<"\n"
#define all(v)        ((v).begin()), ((v).end())
#define pob pop_back()
int main()
{
  // freopen("input.txt", "r", stdin);
  // freopen("output.txt", "w", stdout);
int n;
cin>>n;
int arr[n];
forn cin>>arr[i];
ll ans=0;
forn
{
          for(int j=i+1;j<n;j++)
          {
                    
                    ans+=arr[i]*arr[j];
          }
}
cout<<ans;
return 0;

 
}