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.*;
import java.io.*;
import java.util.stream.Collectors;

public class Main {
  public static void main(String[] args) throws IOException {
    sc = new MScanner(System.in);
    pw = new PrintWriter(System.out);
    int N = sc.nextInt();
    Set<Integer> seen  = new HashSet<>();
    seen.add(N);
    int cnt = 0;
    int num = 0;
    boolean never = false;
    while (true) {
      num = num * 10 + 7;
      cnt ++;
      num = Math.floorMod(num, N);
      if (num== 0) {
        break;
      }
      if(seen.contains(num)) {
        never = true;
        break;
      }
      seen.add(num);
    }
    if(never) {
      pw.println(-1);
    } else  {
      pw.println(cnt);
    }


    //pw.println();

    pw.flush();
    // System.out.println(cnt);
  }

  // sc.close();
  // pw.println(cnt);
  // pw.flush();

  static PrintWriter pw;
  static MScanner sc;

  static class MScanner {
    StringTokenizer st;
    BufferedReader br;

    public MScanner(InputStream system) {
      br = new BufferedReader(new InputStreamReader(system));
    }

    public MScanner(String file) throws Exception {
      br = new BufferedReader(new FileReader(file));
    }

    public String next() throws IOException {
      while (st == null || !st.hasMoreTokens()) st = new StringTokenizer(br.readLine());
      return st.nextToken();
    }

    public int[] intArr(int n) throws IOException {
      int[] in = new int[n];
      for (int i = 0; i < n; i++) in[i] = nextInt();
      return in;
    }

    public long[] longArr(int n) throws IOException {
      long[] in = new long[n];
      for (int i = 0; i < n; i++) in[i] = nextLong();
      return in;
    }

    public int[] intSortedArr(int n) throws IOException {
      int[] in = new int[n];
      for (int i = 0; i < n; i++) in[i] = nextInt();
      shuffle(in);
      Arrays.sort(in);
      return in;
    }

    public long[] longSortedArr(int n) throws IOException {
      long[] in = new long[n];
      for (int i = 0; i < n; i++) in[i] = nextLong();
      shuffle(in);
      Arrays.sort(in);
      return in;
    }

    public Integer[] IntegerArr(int n) throws IOException {
      Integer[] in = new Integer[n];
      for (int i = 0; i < n; i++) in[i] = nextInt();
      return in;
    }

    public Long[] LongArr(int n) throws IOException {
      Long[] in = new Long[n];
      for (int i = 0; i < n; i++) in[i] = nextLong();
      return in;
    }

    public String nextLine() throws IOException {
      return br.readLine();
    }

    public int nextInt() throws IOException {
      return Integer.parseInt(next());
    }

    public double nextDouble() throws IOException {
      return Double.parseDouble(next());
    }

    public char nextChar() throws IOException {
      return next().charAt(0);
    }

    public long nextLong() throws IOException {
      return Long.parseLong(next());
    }

    public boolean ready() throws IOException {
      return br.ready();
    }

    public void waitForInput() throws InterruptedException {
      Thread.sleep(3000);
    }
  }

  static void shuffle(int[] in) {
    for (int i = 0; i < in.length; i++) {
      int idx = (int) (Math.random() * in.length);
      int tmp = in[i];
      in[i] = in[idx];
      in[idx] = tmp;
    }
  }

  static void shuffle(long[] in) {
    for (int i = 0; i < in.length; i++) {
      int idx = (int) (Math.random() * in.length);
      long tmp = in[i];
      in[i] = in[idx];
      in[idx] = tmp;
    }
  }
}