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 Java (OpenJDK 11.0.6) (judge verdict: Wrong Answer)

import java.io.*;
import java.util.*;
class Main
{
	public static void main(String args[]) throws IOException
	{
		try{
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		int n=0;
		n=Integer.parseInt(br.readLine());
		String brr[]=br.readLine().split(" ");
		long ar[]=new long[n];
		for(int i=0;i<n;i++)
		{
			ar[i]=Long.parseLong(brr[i]);
		}
        long sum[] = new long[n]; 
        sum[n - 1] = ar[n - 1]; 
        for (int i = n - 2; i >= 0; i--)  
            sum[i] = sum[i + 1] + ar[i];  
        long res = 0;
        for (int i = 0; i < n - 1; i++)  
            res += (sum[i + 1] * ar[i]); 
        System.out.println((long)(res%(1e9+7)));
    }
	catch(Exception e){}
}}