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 C# (.NET Core 3.1.201) (judge verdict: Runtime Error)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        checked
        {
            var N = int.Parse(Console.ReadLine());
            var A = new List<long>();
            var line = Console.ReadLine().Split(" ");
            for (int i = 0; i < N; i++)
            {
                A.Add(long.Parse(line[i]));
            };

            var sums = new List<long>();
            long sum = 0;
            for (int i = 1; i < N; i++)
            {
                sum += A[i];
                sums.Add(sum);
            }

            sum = sum * A[0];

            for (int left = 0; left < N - 2; left++)
            {
                sum += A[left + 1] * (sums[N - 2] - sums[left]);
                sum = sum % 1000000007;
            }
            Console.WriteLine(sum);
        }


    }
}