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(arg) {
    
    let n = Number(arg.split("\n")[0])
    let a = arg.split("\n")[1].split(" ").map(Number)

    let mod = 10**9+7
    let sum = new Array(n+1)
    sum[0] = a[0]
    for(let i=0; i<n; i++) {
        sum[i+1] = (sum[i]+a[i]) % mod
    }
    //console.log(sum)

    let ans=0
    for(let i=0; i<n; i++) {
        //console.log(`${a[i]} * ${sum[i]} = ${a[i]*sum[i]}`)
        //ans += (a[i]*sum[i])
        ans += ((sum[n]-sum[i+1])%mod) * a[i]
        ans %= mod
    }
    console.log(ans)
    
}

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