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;
using System.Security.Cryptography;

namespace C
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string[] s = Console.ReadLine().Split(' ');
            ulong[] a = new ulong[N];

            #region 自作
            /*for (int i = 0; i < N; i++)
            {
                a[i] = long.Parse(s[i]);
            }

            long ans = 0;

            for (int i = 0; i < N - 1; i++)
            {
                for (int j = i+1; j < N; j++)
                {
                    ans += a[i] * a[j];
                }
            }*/
            #endregion

            #region 解説より
            ulong sum = 0;
            ulong ans = 0;
            ulong mod = 1000000007;

            for (int i = 0; i < N; i++)
            {
                a[i] = ulong.Parse(s[i]);
                sum += a[i];
                sum %= mod;
            }

            for (int i = 0; i < N - 1; i++)
            {
                sum -= a[i];
                if (sum < 0) sum += mod;
                ans += a[i] * sum;

                ans %= mod;
            }
            #endregion

            Console.WriteLine(ans);
        }
    }
}