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.Collections.Generic;

namespace ABC177
{
    public class Program
    {
        public static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            string[] input = Console.ReadLine().Split(" ");
            long[] A = new long[N];
            for(var i = 0; i < N; i++)
            {
                A[i] = long.Parse(input[i]);
            }
            long sum = 0;
            for(var i = 0; i < N - 1; i++)
            {
                for(var j = i+1; j<N; j++)
                {
                    long value = A[i] * A[j];
                    sum += value % 1000000007;
                }
            }

            Console.WriteLine(sum % 1000000007);
        }
    }
}