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: Time Limit Exceeded)

using System;

class Program
{
    static void Main(string[] args)
    {
        int n = int.Parse(Console.ReadLine());
        long[] array = InputLongArray();
        long sum = 0;
        for(int i = 0; i < n; i++)
        {
            for (int j = i + 1; j < n;  j++)
            {
                sum += array[i] * array[j];
                sum %= (1000000000 + 7);

            }
        }
        
        Console.WriteLine(sum);

    }

    public static long[] InputLongArray()
    {
        string[] strs = Console.ReadLine().Split(' ');
       long[] num = new long[strs.Length];
        for (int i = 0; i < num.Length; i++)
        {
            num[i] = long.Parse(strs[i]) % (1000000000 + 7);

        }

        return num;

    }

}