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;

#define ll long long int
#define endl                 "\n"
#define space                " "
#define TLE_na_ho            ios_base::sync_with_stdio(false);cin.tie(NULL)
#define MAX_SIZE             1024
#define MOD                  1000000007
#define pb                   push_back
#define fi                   first
#define se                   second
#define rep(i,a,n)           for(int i=a;i<n;i++)
#define repe(i,a,n)          for(int i=a;i<=n;i++)
#define repr(i,a,n)          for(int i=a;i>=n;i--)
#define ain(arr,n)           for(int i1=0;i1<n;i1++ ) cin>>arr[i1]
#define aout(arr,n)          for(int i1=0;i1<n;i1++ ) cout<<arr[i1]<<space



int main()
{
    TLE_na_ho;
    ll n;
    cin>>n;
    ll a[n];
    ll b[n+1]={0};
    rep(i,0,n){
    	cin>>a[i];
    	b[i+1]+=b[i]+a[i];
    }
    ll ans=0;
    rep(i,0,n)
    {
    	ll sum=b[n]-b[i+1];
    	ans+=a[i]*sum;
    	ans%=MOD;
    }

    cout<<ans;

}