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 Kotlin (1.3.71) (judge verdict: Accepted)

fun main() {
    val n = readInt()
    val a = readInts().map { it.toLong() }
    val mod = 1000000007L

    var total = a.reduce { acc, l -> (acc + l) % mod }
    var ans = 0L
    for (i in 0 until n) {
        total = (mod + total - a[i]) % mod
        ans = (ans + a[i] * total) % mod
    }

    println(ans)
}

// Input Reader
private fun readLn() = readLine()!!

private fun readStrings() = readLn().trim().split(" ")
private fun readInt() = readLn().toInt()
private fun readInts() = readStrings().map { it.toInt() }