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 Perl (5.26.1) (judge verdict: Wrong Answer)

use strict;
my $N=<>;
my $s = <>; chomp $s;
my @A = split(/ /,$s);
my @B;
my $c;
my $d;
my $sum=0;
for($c=0;$c<($N-1);$c=$c+1){for($d=1;$d<($N-$c);$d=$d+1){push(@B,($A[$c]*$A[$c+$d])%((10**9)+7));}}
for($c=0;$c<$N;$c=$c+1){$sum=$sum+$B[$c];}
$sum=$sum%(10**9+7);
print "$sum";