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: Time Limit Exceeded)

((input) => {
  const arr = input.trim().split('\n')[1].split(' ');
  const l = arr.length;
  const n = 1000000007;
  let sum = 0;

  for (let i = 0; i < l - 1; i++) {
    for (let j = i + 1; j < l; j++) {
      sum += arr[i] % n * arr[j] % n;
    }
  }

  console.log(sum % n);
})(require("fs").readFileSync("/dev/stdin", "utf8"));