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 ABC143B
{
    class Input
    {
        /// <summary>
        /// 1行の入力を取得する
        /// </summary>
        /// <returns>文字列</returns>
        public string String()
        {
            return Console.ReadLine();
        }

        /// <summary>
        /// 複数行の入力を取得
        /// </summary>
        /// <returns>文字列の配列</returns>
        public string[] ArrayString(int rowNumber)
        {
            string[] s = new string[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                s[i] = String();
            }

            return s;
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>int型の値</returns>
        public int Int()
        {
            return int.Parse(String());
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>int型の配列</returns>
        public int[] ArrayInt()
        {
            return Split().Select(int.Parse).ToArray();
        }

        /// <summary>
        /// 複数行の入力を取得
        /// </summary>
        /// <param name="rowNumber">行数</param>
        /// <returns>int型の配列</returns>
        public int[] ArrayInt(int rowNumber)
        {
            int[] ints = new int[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                ints[i] = Int();
            }

            return ints;
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>long型の値</returns>
        public long Long()
        {
            return long.Parse(String());
        }

        /// <summary>
        /// 2つの整数が1行に書かれている入力を、2つのlongで受け取る
        /// </summary>
        /// <param name="A"></param>
        /// <param name="B"></param>
        public void Longs(ref long A, ref long B)
        {
            var longs = ArrayLong();
            A = longs[0];
            B = longs[1];
        }

        /// <summary>
        /// 3つの整数が1行に書かれている入力を、3つのlongで受け取る
        /// </summary>
        /// <param name="A"></param>
        /// <param name="B"></param>
        /// <param name="C"></param>
        public void Longs(ref long A, ref long B, ref long C)
        {
            var longs = ArrayLong();
            A = longs[0];
            B = longs[1];
            C = longs[2];
        }
        
        /// <summary>
        /// 4つの整数が1行に書かれている入力を、4つのlongで受け取る
        /// </summary>
        /// <param name="A"></param>
        /// <param name="B"></param>
        /// <param name="C"></param>
        /// <param name="D"></param>
        public void Longs(ref long A, ref long B, ref long C, ref long D)
        {
            var longs = ArrayLong();
            A = longs[0];
            B = longs[1];
            C = longs[2];
            D = longs[3];
        }

        /// <summary>
        /// 2つの整数が複数行に書かれている入力を、2つのlong[]で受け取る
        /// </summary>
        /// <param name="rowNumber"></param>
        /// <param name="A"></param>
        /// <param name="B"></param>
        public void LongsArray(long rowNumber, ref long[] A, ref long[] B)
        {
            A = new long[rowNumber];
            B = new long[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                var l = ArrayLong();
                A[i] = l[0];
                B[i] = l[1];
            }
        }
        
        /// <summary>
        /// 3つの整数が複数行に書かれている入力を、2つのlong[]で受け取る
        /// </summary>
        /// <param name="rowNumber"></param>
        /// <param name="A"></param>
        /// <param name="B"></param>
        /// <param name="C"></param>
        public void LongsArray(long rowNumber, ref long[] A, ref long[] B, ref long[] C)
        {
            A = new long[rowNumber];
            B = new long[rowNumber];
            C = new long[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                var l = ArrayLong();
                A[i] = l[0];
                B[i] = l[1];
                C[i] = l[2];
            }
        }
        
        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>long型の配列</returns>
        public long[] ArrayLong()
        {
            return Split().Select(long.Parse).ToArray();
        }

        public long[] ArrayLong(int rowNumber)
        {
            long[] longs = new long[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                longs[i] = Long();
            }

            return longs;
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>double型の値</returns>
        public double Double()
        {
            return double.Parse(String());
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>double型の配列</returns>
        public double[] ArrayDouble()
        {
            return Split().Select(double.Parse).ToArray();
        }

        private IEnumerable<string> Split()
        {
            return String().Split(' ');
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Solver solver = new Solver();
            solver.Solve();
        }
    }

    class Solver
    {
        public Solver()
        {
            Input input = new Input();
            N = input.Long();
            D = input.ArrayLong();
        }

        private long N;
        private long[] D;

        public void Solve()
        {
            long ans = 0;
            for (int i = 0; i < N; i++)
            {
                for (int j = i+1; j < N; j++)
                {
                    ans += D[i] * D[j];
                }
            }
            Console.WriteLine(ans);
        }
    }
}