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 Go (1.6) (judge verdict: Wrong Answer)

package main

import "fmt"

func main() {
 n := 0
 hp := 0
 //たこ焼きの個数を受け取る
 fmt.Scanf("%d",n)
 
 //たこ焼きのおいしさを配列に格納
 num := make([]int,n)
 for i:=0; i < n; i++ {
 	fmt.Scanf("%d",num[i])
 }

 for i:=0; i < n-1; i++ {
   for j:=i+1; j<n; j++ {
     hp += num[i] * num[j]
   }
 }
 fmt.Printf("%d",hp)
}