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;
using System.Linq;
using System.Collections.Generic;
using System.Text.RegularExpressions;

namespace AtCoderBeginnerContest177
{
    class Program
    {
        static void Main(string[] args)
        {

            // 課題C：Sum of product of pairs(2020/8/29)

            int num = int.Parse(Console.ReadLine());
            string[] numSet = Console.ReadLine().Split(' ');

            long ans = 0;
            int modValue = 1000000007;

            for (int i = 0; i < num; i++)
            {
                long sum = 0;
                
                for (int j = i + 1; j < num; j++) sum = (sum + int.Parse(numSet[j])) % modValue;

                ans = ans + int.Parse(numSet[i]) * sum % modValue;
            }

            Console.Write(ans % modValue);
        }

    }
}