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: Wrong Answer)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Main {
  public static void main(String [] args) {
    System.out.println(solve());
  }
  
  public static long solve() {
    FastReader fr = new FastReader();
    int N = fr.nextInt();
    int[] d = new int[N];
    long sum = 0;
    long power = 0;
    for (int i = 0; i < N; i++) {
      d[i] = fr.nextInt();
      sum += d[i];
      power += d[i] * d[i];
    }
    
	return sum * sum - power;
  }
}

class FastReader {
  BufferedReader br;
  StringTokenizer st;

  public FastReader() {
    br = new BufferedReader(new
        InputStreamReader(System.in));
  }

  String next() {
    while (st == null || !st.hasMoreElements())
    {
      try
      {
        st = new StringTokenizer(br.readLine());
      }
      catch (IOException e)
      {
        e.printStackTrace();
      }
    }
    return st.nextToken();
  }

  int nextInt() {
    return Integer.parseInt(next());
  }

  long nextLong() {
    return Long.parseLong(next());
  }

  double nextDouble() {
    return Double.parseDouble(next());
  }

  String nextLine() {
    String str = "";
    try {
      str = br.readLine();
    }
    catch (IOException e) {
      e.printStackTrace();
    }
    return str;
  }
}