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: Accepted)


import scala.io.StdIn

object Main extends App {
  StdIn.readLine()
  val nums = StdIn.readLine().split(' ').map(BigInt(_))

  val sum = nums.sum

  val tmp1 = sum*sum - nums.map(n=>n*n).sum

  val ans = (tmp1/2) % (BigInt((math.pow(10L,9L)+7).toLong))

  println(ans)
}