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) {
  const lines = input.split('\n');
  const N = lines[0];
  const A = lines[1].split(' ').map(i => parseInt(i));
  const D = Math.pow(10, 9) + 7;
  const notes = [];
  let sum = 0;
  A.reverse().map(a => {
    sum += a;
    sum = sum % D;
    notes.push(sum);
  });
  notes.reverse();
  A.reverse();
  console.log(A);
  console.log(notes);
  sum = 0;
  A.forEach((a, i) => {
    if (i !== N - 1) {
      console.log(a, notes[i+1]);
      sum += (a * notes[i+1]) % D;
    }
  });
  console.log(sum % D);
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));