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.*;
import static java.lang.System.*;

public class Main {
    static Scanner sc = new Scanner(System.in);

    public static void main(String[] args) {
        int n = sc.nextInt();
        int b = 10;
        int c = 0;
        int count = 1;
        double x = 10d;
        double y = 7d;

        for (int a = 7;; count++) {
            c = ((int) (a * (Math.pow(b, count) - 1)) / (b - 1));
            if (c % n == 0) {
                System.out.println(count);
                break;
            } else if (n % 2 == 0) {
                System.out.println("-1");
                break;
            
            } else if(c > Math.pow(x,y)){
                System.out.println(n-1);
                break;
            }
        }

    }
}