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: Accepted)

import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.Scanner;

public class Main {
    public static class Set {
        int a,b;
        Set(int a, int b) {
            this.a = a;
            this.b = b;
        }

        public boolean isSamePattern(int a, int b) {
            if (this.a == a) if (this.b == b) return true;
            if (this.a == b) if (this.b == a) return true;
            return false;
        }
    }
    public static void main(String[] args) {
        ArrayList<Byte> Takoyaki = new ArrayList<>();
        readLine();
        for (String s : readLine().split(" ")) {
            Takoyaki.add(Byte.parseByte(s));
        }

        //Set, result
        LinkedHashMap<Set, Integer> Map = new LinkedHashMap<>();
        for (int index1 = 0; index1 < Takoyaki.size(); index1++) {
            for (int index2 = 0; index2 < Takoyaki.size(); index2++) {
                if (index1 == index2) continue;
                int Takoyaki1 = Takoyaki.get(index1);
                int Takoyaki2 = Takoyaki.get(index2);

                boolean flag = false;
                for (Set s : Map.keySet()) {
                    if (s.isSamePattern(index1, index2)) flag = true;
                }
                if (flag) continue;

                Map.put(new Set(index1, index2), Takoyaki1 * Takoyaki2);
            }
        }

        int result = 0;
        for (int i : Map.values()) result += i;
        Print(result);

    }

    private static final Scanner __in = new Scanner(System.in);
    private static String readLine() {
        return __in.nextLine();
    }

    private static void Print(int m) {
        System.out.println(m);
    }
}