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# (Mono-mcs 6.8.0.105) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var n = int.Parse(Console.ReadLine());

        if(n % 2 == 0)
        {
            Console.WriteLine("-1");
        }
        else
        {
            var ans = 0;
            var rem = 0;
            var current = 0;
            var currentDigit = 1;
            for (var i = 1;i <= 1000000; i++)
            {
                if(currentDigit == 1)
                {
                    if(7 % n == 0)
                    {
                        ans = 1;
                        break;
                    }
                    current = 7 % n;
                    currentDigit += 1;
                    rem = 7 % n;
                }
                else
                {
                    if ((rem + ((current * (10 % n))) % n) % n == 0)
                    {
                        ans = currentDigit;
                        break;
                    }
                    rem = (rem + ((current * (10 % n))) % n) % n;
                    current = (current * (10 % n)) % n;
                    currentDigit += 1;
                }
            }

            Console.WriteLine(ans);
        }



        Console.ReadLine();
    }


}