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.Text;
using System.Threading.Tasks;
 
namespace AtCoder
{
    class ABC177C
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            string[] str = Console.ReadLine().Split(' ');
            int[] a = new int[n];
            long mod = 1000000007;
            for(int i = 0; i < n; i++)
            {
                a[i] = int.Parse(str[i]);
            }
            long ans = 0;
            long sum = 0;
            for(int i = 1; i < n; i++)
            {
                sum += a[i]%mod;
            }
            for(int i = 1; i < n; i++)
            {
                ans += (a[i-1]%mod)*(sum%mod);
                sum -= a[i];
            }
            Console.WriteLine(ans%mod);
        }
    }
}