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>
#define rep(i,n) for(long long i=0; i<n; i++)
#define Rep(i,n) for(long long i=1; i<n; i++)
#define ll long long
#include <math.h>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <queue>
#include <set>
#include <vector>


using namespace std;
#define PI acos(-1)
using P =pair<ll,ll>;

ll ketasu(ll a){
   ll num=1;
   while(a/10){
       num++;
       a/=10;
   }
   return num;
}

ll gcd(ll a,ll b){return b ? gcd(b,a%b) :a;}
ll lcm(ll a,ll b){return a*b/gcd(a,b);}

ll kosuu(ll a){
    ll sum=0;
    for(ll i=1;i*i<=a;i++){
        if(a%i==0){
            if(a!=1&&i*i!=a){
            sum+=2;
            }else{
             sum++;   
            }
        }
    }
    return sum;
}

ll n;



    vector<pair<ll,ll>>p;
void fs(ll a){
    for(ll i=2;i*i<=n;i++){
        ll cnt=0;
        while(n%i==0){
            n/=i;
            cnt++;
        }
        p.emplace_back(i,cnt);
    }
    return;
}


ll di[]={1,0,-1,0};
ll dj[]={0,-1,0,1};
int main(){
   int n;
   cin>>n;
    long long int a[n],ans=0,i;
    int mod=1000000007;
    long long int sum=0;
 
    for( i = 0 ; i < n ; i++ ){
            cin>>a[i];
        sum = sum + a[i];
    }
    for( i = 0 ; i < n ; i++ ){
        sum = sum - a[i];
 
        ans+=(a[i]%mod * sum%mod)%mod;
    }
    cout<<ans%mod;
 
    return 0;
}