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 Ruby (2.3.3) (judge verdict: Accepted)

def main
  delicious_degrees = []

  object_count = gets().to_i

  return_flg = false
  gets.split(" ").each do |value|
    value.to_i >= 0 ? delicious_degrees << value.to_i : return_flg = true
    next if return_flg
  end

  return if object_count < 2 || object_count > 50
  return if return_flg

  total_delicious_value = 0
  delicious_degrees.each_with_index do |degree, index|
    next if index == object_count
    total_delicious_value += total_of_multiplication( degree, delicious_degrees[index+1..object_count] )
    # p "#{index}要素目 値:#{degree} 合計:#{total_delicious_value}"
  end
  
  p total_delicious_value
end

def total_of_multiplication( coefficient, targets )
  total = 0
  targets.each do |v|
    total += coefficient * v
  end
  total
end

main