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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
 
namespace Sample
{
  static class Sample
  {
    static void Main()
    {
        int n = int.Parse(Console.ReadLine());
        string[] input = Console.ReadLine().Split(' ');
        List<double> numList = new List<double>();
        double modnum = Math.Pow(10,9) + 7;

        foreach (string s in input) {
            numList.Add(double.Parse(s) % modnum);
        }


        double count = 0;

        while(numList.Count != 0){
            double subcount = numList.Sum() % modnum;
            count += numList[0] * (subcount - numList[0]);
            numList.RemoveRange(0,1);
        }

        Console.WriteLine(Convert.ToString(count % modnum));
    }
  }
}