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()
{
  unsigned long n;
  scanf("%lu",&n);
  unsigned long a[n];
  int i;
  i = 0;
  unsigned long sum = 0;
  unsigned long p = 0;
  while (i < n)
  {
    scanf("%lu",&a[i]);
    if (i != 0)
    {
        p += a[i - 1];
    	sum += a[i]*p;
    }
    i++;
  }
  printf("%lu",sum % (1000000000 + 7));
}