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 Java8 (OpenJDK 1.8.0) (judge verdict: Runtime Error)

import java.util.Scanner;

public class Main
{
    public static void main(String[] args)
    {
        Scanner in1 = new Scanner(System.in);
        int N = in1.nextInt();
        int D[][]=new int [N][2];
        Scanner d = new Scanner(System.in);
        int sum=0;
        for (int x=0;x<N;x++)
        {
            D[x][0]=d.nextInt();
        }
        D[N-1][1]=1;
        for (int x=N-2;x>=0;x--)
        {
            D[x][1]=D[x+1][0];
            if (x<N-2)
            {
                D[x][1]+=D[x+1][1];
            }
        }
        for (int x=0;x<N-1;x++)
        {
            sum+=D[x][0]*D[x][1];
        }
        System.out.print(sum);
    }
}