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 Java (OpenJDK 11.0.6) (judge verdict: Wrong Answer)

import java.util.Scanner;

public class Main {

  public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);
    int k = sc.nextInt();
    int p = 7 % k;
    int np = 0;
    boolean[] b = new boolean[k];
    b[p] = true;
    int cnt = 1;

    if (k == 7) {
      System.out.println(1);
      return;
    }

    while (true) {
      cnt++;
      np = ((p * 10) + 7) % k;
      if (np == 0) {
        System.out.println(cnt);
        return;
      } else if (b[np]) {
        System.out.println(-1);
        return;
      } else {
        b[np] = true;
        p = np;
      }
    }

  }
}