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;
using System.Collections.Generic;
using System.Linq;
using static System.Console;

public static class Program
{
    static long k;
    public static void Main()
    {
        k = long.Parse(ReadLine());
        if(k % 2 == 0 || k % 5 == 0)
        {
            WriteLine(-1);
            return;
        }
        long left = 0;
        long res = 1;
        for(int i = 1; ; ++i)
        {
            left = (left + res * 7) % k;
            if (left == 0)
            {
                WriteLine(i);
                return;
            }
            res *= 10;
            res %= k;
        }
    }
}