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.Linq;
using System.Text;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Diagnostics;
using System.Threading;
using System.Runtime.CompilerServices;
using System.IO.Pipes;

namespace AtCoder
{
    class Program
    {

        struct Point
        {
            public int x;
            public int y;

            public Point(int x, int y)
            {

                this.x = x;
                this.y = y;

            }

        }


        const int BiggestMod = 1000000007;

        public static void Main()
        {

            int num = int.Parse(Console.ReadLine());
            long ans = 0;

            long[] args = Console.ReadLine().Split(' ').Select(x => long.Parse(x)).ToArray();

            long[] data = new long[num - 1];

            data[num - 2] = args[num - 1];

            for (int i = num - 2; 0 < i; --i)
                data[i - 1] = args[i] + data[i];

            for (int i = 0; i < num - 1; ++i)
            {

                ans += args[i] * data[i];
                ans %= BiggestMod;

            }

            Console.WriteLine(ans);

        }

    }

}