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.util.Scanner;
public class Main {
	
		public static void main(String[] args) {
			try(Scanner sc = new Scanner(System.in)){
				int n = sc.nextInt();
				long[] s = new long[n];
				long[] f = new long[n+1];
				int mod = 1000000007;
				f[0] = 0;
				 long d = 0;
				for(int x = 0;x < n;x++) {
					s[x] = sc.nextInt();
				}
				for(int x = 1;x <= n;x++) {
					f[x] = f[x-1]+s[x-1];
				}
				for(int x = 0;x < n;x++) {
					long sum = f[n] - f[x + 1];
					d += s[x] * sum; 
					d = d % mod;
				}
				System.out.println(d);
			}
		}
}