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: Wrong Answer)

using System;
using System.IO;

class Program
{
    static void Main(string[] args) {
        var scan = new Scanner();
        var writer = new StreamWriter(Console.OpenStandardOutput()) {AutoFlush = false};
        Console.SetOut(writer);
        Console.WriteLine(Solve(scan));
        Console.Out.Flush();
    }
    static object Solve(Scanner scan) {
        int K = scan.NextInt();
        var temp = 0;
        for (int i = 1; i < K; i++) {
            temp = (10 * temp + 7) % K;
            if (temp == 0) return i;
        }
        return -1;
    }
}

class Scanner
{
    string[] s;
    int i;
    public Scanner() {
        s = new string[0];
        i = 0;
    }
    public string Next() {
        if (i < s.Length) return s[i++];
        string str = Console.ReadLine();
        while (str == "") str = Console.ReadLine();
        s = str.Split(' ', StringSplitOptions.RemoveEmptyEntries);
        if (s.Length == 0) return Next();
        i = 0;
        return s[i++];
    }
    public int NextInt() => int.Parse(Next());
}