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 Ruby (2.7.1) (judge verdict: Accepted)

require "set"

def max(a, b); a < b ? b : a; end
def min(a, b); a < b ? a : b; end

class Problem
  attr_accessor *(?a..?z).to_a.map(&:to_sym)

  def initialize
    @k = gets.to_s.to_i
    @cnt = Set.new
  end

  def solve
    i = 1
    n = 7
    loop do
      return i if n % k == 0
      n = n * 10 + 7
      n %= k
      i += 1
      return -1 if @cnt.include?(n)
      @cnt << n
    end
  end

  def show(ans)
    puts ans
  end
end

Problem.new.instance_eval do
  show(solve)
end