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)

object Main extends App {
  val mod = 1000000007
  val sc = new java.util.Scanner(System.in) 
  val n = sc.nextInt()
  val a = Array.fill(n)(sc.nextLong())
  val cumSum = a.scanLeft(0L)((x, y) => x + y).tail
  val ans = for(i <- 0 until n) yield {
    a(i) * (cumSum(cumSum.length-1) - cumSum(i)) % mod
  }
  println(ans.sum % mod)
}