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

// Solution for https://atcoder.jp/contests/abc174/tasks/abc174_c

import (
	"bufio"
	"fmt"
	"io"
	"os"
)

type contest struct {
	in  io.Reader
	out io.Writer
}

func (con *contest) Scan(a ...interface{}) (int, error) {
	return fmt.Fscan(con.in, a...)
}
func (con *contest) Scanln(a ...interface{}) (int, error) {
	return fmt.Fscanln(con.in, a...)
}
func (con *contest) Scanf(f string, a ...interface{}) (int, error) {
	return fmt.Fscanf(con.in, f, a...)
}
func (con *contest) Print(a ...interface{}) (int, error) {
	return fmt.Fprint(con.out, a...)
}
func (con *contest) Println(a ...interface{}) (int, error) {
	return fmt.Fprintln(con.out, a...)
}
func (con *contest) Printf(f string, a ...interface{}) (int, error) {
	return fmt.Fprintf(con.out, f, a...)
}
func main() {
	in := bufio.NewReader(os.Stdin)
	out := bufio.NewWriter(os.Stdout)
	defer out.Flush()
	con := &contest{in: in, out: out}
	con.main()
}

func (con *contest) main() error {
	var k int
	con.Scan(&k)
	c, x := 0, 0
	max := 10000000
	for c < max {
		x = x*10 + 7
		x %= k
		c++
		if x == 0 {
			break
		}
	}
	if c == max {
		con.Println(-1)
	} else {
		con.Println(c)
	}
	return nil
}