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: Accepted)

my $N=<>;
my $s=<>;chomp $s;
my @A = split(/ /,$s);
my $mod = 10**9+7;


my $sum =0;
my $cal =0;

for(my $i=0;$i<=($#A-1);$i++ ){
    
    $cal = ( $cal + $A[$i])%$mod;
    $sum = ($sum +  $cal * $A[$i+1])%$mod;
     
}

print "$sum";