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 Scala (2.13.1) (judge verdict: Wrong Answer)

import scala.io.StdIn._
import scala.math._
import scala.util.control.Breaks

object Main extends App with StdInHelper {
  val n = readInt
  val as = readLongs

  var result = 0L
  as.combinations(2)
    .map { case Array(a, b) => (a * b) % 1000000007L }
    .foreach(a => result = (result + a) % 1000000007L)

  println(result)
}

trait StdInHelper {
  def readStrings = {
    val l = readLine
    if (l == "") Array.empty[String]
    else l.split(" ")
  }
  def readInts = readStrings.map(_.toInt)
  def readLongs = readStrings.map(_.toLong)
  def readDoubles = readStrings.map(_.toDouble)
  def readBigInts = readStrings.map(BigInt.apply(_))
  def readBigDecimals = readStrings.map(BigDecimal.apply(_))
}