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# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            //[summary]B - TAKOYAKI FESTIVAL 2019
            int N = int.Parse(Console.ReadLine());
            var d = ReadLine();

            var lib = new Library.Combination();
            var combis = lib.AllCombinations<int>(d, 2);
            int sum = 0;

            foreach(var c in combis)
            {
                sum += c[0] * c[1];
            }

            Console.WriteLine(sum);
        }

        static List<int> ReadLine()
        {
            var line = Console.ReadLine();
            var arr = line.Split(' ');

            return arr.Select(x => int.Parse(x)).ToList();
        }
    }
}

namespace Library
{
    class Combination
    {
        public IEnumerable<List<T>> AllCombinations<T>(List<T> source, int n,
                                                       bool isAllowDupication = false)
        {
            //[summary]異なる要素から、n個を選ぶ組み合わせを列挙する
            //"isAllowDupication"を"true"にすると、重複を許可する
            //(重複を許可して、nを要素数と同じにすると、順列を列挙する)

            if (n == 1)
            {
                foreach (var s in source)
                {
                    yield return new List<T> { s };
                }

                yield break;
            }
            else
            {
                for (int i = 0; i < source.Count; i++)
                {
                    List<T> clone;

                    if (isAllowDupication)
                    {
                        //重複を許可する場合は、その要素のみを削除
                        clone = new List<T>(source);
                        clone.RemoveAt(i);
                    }
                    else
                    {
                        //重複を許可しない場合は、自分よりも前の要素はすべて削除
                        clone = new List<T>(source.Skip(i + 1));
                    }

                    //再帰処理
                    var items = AllCombinations(clone, n - 1, isAllowDupication);

                    foreach (var item in items)
                    {
                        item.Insert(0, source[i]);
                        yield return item;
                    }
                }
            }
        }
    }
}