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 (Clang 10.0.0) (judge verdict: Wrong Answer)

#include <stdio.h>


int main(void){
  int i, j ,n;
  long long ans;
  scanf("%d", &n);
  int a[n];
  for(i = 0; i < n; i++){
    scanf("%d", &a[i]);
  }
  
  long cnt = 0;
  for(i = 0; i < n - 1; i++){
    cnt += a[i];
    ans += a[i+1] * cnt;
  }
  ans = ans % 1000000007;

  printf("%lld\n", ans);

  return 0;
}