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() {
    abc177c()
}

fun abc177c() {
    val n = readLine()!!.toInt()
    val a = readLine()!!.split(" ").map { it.toLong() }
    var ans = 0L
    val base = 1_000_000_007L
    var p = a[0] % base
    for (i in 1 until n) {
        val t = (p * a[i]) % base
        ans = (ans + t) % base
        p = (p + a[i]) % base
    }
    println(ans)
}