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.Linq;
using System.Text;
using System.Threading.Tasks;

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

            if (K % 2 == 0)
            {
                Console.WriteLine("-1");
                return;
            }

            var seven = "7";
            var rtn = 1;

            for (int i = 0; i < 19; i++)
            {
                long tmp = long.Parse(seven);

                if (tmp % K == 0)
                {
                    Console.WriteLine(rtn);
                    break;
                }

                rtn++;
                seven += "7";
            }

            Console.WriteLine("-1");
        }
    }
}