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-csc 3.5.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;

class Test
{

    public static void Main()
    {

        var n = int.Parse(Console.ReadLine());

        List<int> ans = new List<int>();
        ans.Add(7);
        ans.Add(77);
        ans.Add(777);
        ans.Add(7777);
        ans.Add(77777);
        ans.Add(777777);
        ans.Add(7777777);
        ans.Add(77777777);
        ans.Add(777777777);
        int i = 1;
        int tmp = n;
        bool ch = false;
        while (i < 1000000)
        {
            tmp = n * i;
            i++;
            if (ans.Contains(tmp))
            {
                Console.Write(ans.IndexOf(tmp)+1);
                ch = true;
                break;
            }

        }
        if(!ch)
        {
            Console.Write(-1);
        }

    }
}