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 C# (.NET Core 3.1.201) (judge verdict: Accepted)

using System.Collections.Generic;
using System;

public class Hello
{
    public static void Main()
    {
        var k = int.Parse(Console.ReadLine().Trim());
        getAns(k);
    }
    static void getAns(int k)
    {
        if (k % 2 == 0) { Console.WriteLine(-1); return; }
        var hs = new HashSet<int>();
        var a = 7;
        var p = 1;
        while (true)
        {
            a %= k;

            if (a == 0) { Console.WriteLine(p); return; }
            else
            {
                if (!hs.Add(a)) { Console.WriteLine(-1); return; }
            }
            a = a * 10 + 7;
            p++;
        }
    }
}