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: Accepted)

import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int k = sc.nextInt();
        
        int now = 7;
        now %= k;
        Set<Integer> set = new HashSet<Integer>();
        int count = 1;
        
        int oldsize = 0;
        while (true) {
            if (now == 0) break;
            
            now *= 10;
            now += 7;
            now %= k;
            
            count++;
            
            set.add(now);
            if (oldsize == set.size()) {
                System.out.println(-1);
                return;
            }
            oldsize = set.size();
        }
        System.out.println(count);
    }
}