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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCoder
{
    class Program
    {
        static long ReadLong()
        {
            return long.Parse(Console.ReadLine());
        }

        static long[] ReadLongs()
        {
            return Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
        }

        static double ReadDouble()
        {
            return double.Parse(Console.ReadLine());
        }

        static double[] ReadDoubles()
        {
            return Console.ReadLine().Split(' ').Select(double.Parse).ToArray();
        }

        static string ReadString()
        {
            return Console.ReadLine();
        }

        static string[] ReadStringSplit()
        {
            return Console.ReadLine().Split(' ');
        }

        static void A()
        {
            var AB = ReadLongs();
            var A = AB[0];
            var B = AB[1];
            Console.WriteLine(Math.Max(A - (2 * B), 0));
        }

        static void B()
        {
            var N = ReadLong();
            var ds = ReadLongs();
            long ans = 0;
            foreach (var x in Enumerable.Range(0, (int)N))
            {
                foreach (var y in Enumerable.Range(x, (int)N - x))
                {
                    if (x == y)
                    {
                        continue;
                    }
                    ans += ds[x] * ds[y];
                }
            }
            Console.WriteLine(ans);
        }

        static void C()
        {

        }

        static void D()
        {

        }

        static void Main(string[] args)
        {
            //A();
            B();
            //C();
            //D();
        }
    }
}