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>
#include<stdlib.h>
#include<string.h>
#define rep(i,l,r) for(int i=l;i<r;i++)
#define MOD 1000000007
typedef long long ll;

int main() {
	ll n,a[200001];
	scanf("%lld",&n);
	ll sum = 0;
	rep(i, 0, n) {
		scanf("%lld", &a[i]);
		sum += a[i];
	}
	ll ans = 0;
	rep(i, 0, n) {
		sum -= a[i];
		sum %= MOD;
		ans += (a[i] * sum)%MOD;
		ans %= MOD;
	}
	printf("%lld", ans);
	return 0;
}