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)

(async () => {
    let input = '';
    for await (const chunk of process.stdin) input += chunk;
    let values = input.split(/\s/).map(v => +v);
    
    const length = values.shift();
    values = values.slice(0, length);

    let sum = 0;
    let tmp = 0;
    for (let i = length - 2; i >= 0; i--) {
        tmp += values[i + 1];
        sum = (sum + values[i] * tmp) % (1e9 + 7);
    }

    console.log(sum % (1e9 + 7));

})();