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 JavaScript (Node.js 12.16.1) (judge verdict: Wrong Answer)

function Main(input) {
  input = input.split("\n");
  const N = parseInt(input[0], 10);
  const A = input[1].split(" ").map(n => parseInt(n, 10));

  var count = 0;
  var sum = 0;

  A.forEach(function(a) {
    sum += a;
  });

  for(var i = 0; i < (N-1); i++){
    sum-=A[i]
    count+=A[i]*sum
    count = count%1000000007
  }
  
  console.log(count);
  
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));