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 <bits/stdc++.h>
using namespace std;
 
int main() {
  int n;
  cin >> n;
  vector<long long>a(n);
  long long sum1=0;
  for (int i=0;i<n;i++) {
    cin >> a.at(i); 
    sum1+=a.at(i);
  }
  sum1%=1000000007;
  long long ans=0;
  for (int i=0;i<n-1;i++) {
    sum1-=a.at(i);
    sum1%=1000000007;
    ans+=a.at(i)*sum1;
    ans%=1000000007;
  }
  cout << ans << endl;
}