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 (Clang 3.8.0) (judge verdict: Wrong Answer)

#include <stdio.h>

int main(void){
  int N = 0; // たこ焼きの個数
  int d[50]; // おいしさの値を格納する配列
  int heal_points_list[1225]; // 回復量を格納する配列
  int sum_heal_points = 0; // 回復量の総和

  // たこ焼きの個数を設定する
  scanf("%d", &N);

  // たこ焼き達においしさ値を設定する
  for(int i = 0;i < N; i++) {
    scanf("%d", &d[i]);
  }

  // 総当たり方式の実行回数を求める
  int calc_count = (N * (N - 1)) / 2;

  // 各たこ焼き同士の回復量を総当たり方式で計算する
  for(int i = 0; i < N; i++) {
    int num = i + 1;

    // i が配列の要素数と同じかそれ以上になった場合、numに ０を代入する
    if(calc_count <=  i + 1) num = 0;

    heal_points_list[i] = d[i] * d[num];
  }

  // 回復量の総和を計算する
  for(int i = 0; i < calc_count; i++) {
    sum_heal_points += heal_points_list[i];
  }

  // 出力
  printf("%d\n", sum_heal_points);

  return 0;
}