Problem:
Score : 300 points

Problem Statement

Given are N integers A_1,\ldots,A_N.

Find the sum of A_i \times A_j over all pairs (i,j) such that 1\leq i < j \leq N, modulo (10^9+7).

Constraints

2 \leq N \leq 2\times 10^5

0 \leq A_i \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
A_1 \ldots A_N

Output

Print \sum_{i=1}^{N-1}\sum_{j=i+1}^{N} A_i A_j, modulo (10^9+7).

Sample Input 1

3
1 2 3

Sample Output 1

11

We have 1 \times 2 + 1 \times 3 + 2 \times 3 = 11.

Sample Input 2

4
141421356 17320508 22360679 244949

Sample Output 2

437235829

Solution in PHP (7.4.4) (judge verdict: Wrong Answer)

<?php
//Sの部分文字列になりうる全パターンに対して一致率を算出して一致率が高いパターンに関して
//Tが部分文字列となるのに必要な文字の変更数を算出する
fscanf(STDIN,"%d",$n);
$sum = 0;
$a_li = explode(" ",trim(fgets(STDIN)));
$sum_ini = array_sum($a_li);
for($i=0;$i<$n-1;$i++){
 $sum_ini -= $a_li[$i];
 $sum += $a_li[$i]*($sum_ini);
}

echo ($sum%(10**9+7));