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;

namespace AtBC177
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            string[] str = Console.ReadLine().Split();
            double[] ary = new double[n];
            double output = 0;
            for (int i = 0;i < n; i++)
            {
                ary[i] = double.Parse(str[i]);
            }
            for (int j = 0;j < n - 1; j++)
            {
                for (int k = j + 1;k < n; k++)
                {
                    output += ary[j] * ary[k] % 1000000007 ;
                    output %= 1000000007 ;
                }
            }
            Console.WriteLine(output);
        }
    }
}