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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#define Fori(g) for(int i=0;i<g;i++)
#define	Forj(g) for(int j=0;j<g;j++)
#define min(aa,bb) (aa<bb)?aa:bb
#define max(aa,bb) (aa<bb)?bb:aa
//#define gcd(aa,bb) return (!bb)?aa:gcd(bb,aa%bb)

int main(){
	int n,sum=0;
	scanf("%d",&n);
	int d[n];
	Fori(n)scanf("%d",&d[i]);
	for(int i=0;i<n;i++){
		for(int j=i+1;j<n;j++){
			sum+=d[i]*d[j];
		}
	}
	printf("%d\n",sum);
	return 0;
}