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: Wrong Answer)

import kotlin.math.*
import java.util.*
import java.io.*
val big = 1000000007L
var ans:Long = 0L
var sum:Long = 0L
var powe:Long = 0L
fun main() {
    val N = readLine()!!.toLong()
    val A = readLine()!!.split(" ").map { it.toLong() }
    val temp = N.toInt()

    for(i in 0..temp-1){
        sum = (sum + A[i])%big
    }
    for(j in 0..temp-1){
        powe = (powe + ((A[j]*A[j])%big))%big
    }
    ans = ((sum*sum)%big)-powe
    if(ans < 0){
        ans = ans + big
    }
    ans = ans/2
    println(ans)
}