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 (
	"bufio"
	"fmt"
	"log"
	"os"
	"strconv"
	"sync"
)


func main() {
	n := scanSplitedInt()
	array := make([]uint64, 0, n)

	// 配列を作る
	for i := 0; i < n; i++ {
		array = append(array, scanSplitedUint64())
	}

	// 累積和を作る
	cusum := make([]uint64, n+1)
	for i := 0; i < n; i++ {
		cusum[i+1] = cusum[i] + array[i]
	}

	var sum uint64
	for i, v := range array {
		sum += (v * (cusum[n] - cusum[i+1])) % 1000000007
	}

	fmt.Println(sum)
}


var once sync.Once
var scanner *bufio.Scanner

func scan() {
	once.Do(func() {
		scanner = bufio.NewScanner(os.Stdin)
		scanner.Split(bufio.ScanWords)
	})
	scanner.Scan()
}

func check(err error) {
	if err != nil {
		log.Fatal(err)
	}
}

func scanSplitedUint64() uint64 {
	scan()
	val, err := strconv.ParseUint(scanner.Text(), 10, 64)
	check(err)
	return val
}

func scanSplitedInt() int {
	scan()
	val, err := strconv.Atoi(scanner.Text())
	check(err)
	return val
}