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 (GCC 9.2.1) (judge verdict: Wrong Answer)

#include <stdio.h>

int main(){
   int N, i;
   long int sum=0;
   scanf("%d", &N);
   long int a[N], s[N];
   for(i=0; i<N; i++) scanf("%ld", &a[i]);
   s[0]=a[0];
   for(i=1; i<N; i++) s[i]=s[i-1]+a[i];
   for(i=0; i<N-1; i++) sum=(sum+a[i]*(s[N-1]-s[i]))%1000000007;
   printf("%ld", sum);
}