Problem:
Score : 300 points

Problem Statement

Takahashi loves the number 7 and multiples of K.

Where is the first occurrence of a multiple of K in the sequence 7,77,777,\ldots? (Also see Output and Sample Input/Output below.)

If the sequence contains no multiples of K, print -1 instead.

Constraints

1 \leq K \leq 10^6

K is an integer.

Input

Input is given from Standard Input in the following format:

K

Output

Print an integer representing the position of the first occurrence of a multiple of K. (For example, if the first occurrence is the fourth element of the sequence, print 4.)

Sample Input 1

101

Sample Output 1

4

None of 7, 77, and 777 is a multiple of 101, but 7777 is.

Sample Input 2

2

Sample Output 2

-1

All elements in the sequence are odd numbers; there are no multiples of 2.

Sample Input 3

999983

Sample Output 3

999982

Solution in Go (1.14.1) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

func main() {
	sc.Split(bufio.ScanWords)
	k := nextInt()
	c := 0
	for i := 0; i < k; i++ {
		c = c*10 + 7
		if c%k == 0 {
			fmt.Println(i + 1)
			return
		}
		c %= k
	}
	fmt.Println(-1)
}

func nextInt() int {
	i, err := strconv.Atoi(nextWord())
	pnc(err)
	return i
}

func nextWord() string {
	sc.Scan()
	return sc.Text()
}

func pnc(err error) {
	if err != nil {
		panic(err)
	}
}