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 Go (1.14.1) (judge verdict: Wrong Answer)

package main
  
import (
        "fmt"
)

func main() {
        var (
                i, s, N, ans int64
                mod          int64 = 1000000007
        )
        fmt.Scanf("%d", &N)
        arr := make([]int64, N)
        for i = 0; i < N; i++ {
                fmt.Scanf("%d", &arr[i])
                s = s%mod + arr[i]
        }
        for i = 0; i < N; i++ {
                s = (s - arr[i]) % mod
                ans += (arr[i] * s) % mod
        }
        fmt.Println(ans % mod)
}