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

namespace AtCoder.ABC
{
	class QuestionC
	{
		public static void Main(string[] args)
		{
            var s = Console.ReadLine();
            var n = int.Parse(s);

            if(n % 2 == 0 || n % 5 == 0) {
                Console.WriteLine(-1);
                return;
            }

            var ret = 1;

            var sw = new Stopwatch();
            sw.Start();

            var val = 7;
            var mul = 7;

			while (true) {
                if(val % n == 0) {
                    break;
				}

                if(sw.ElapsedMilliseconds >= 1900) {
                    Console.WriteLine(-1);
                    return;
				}

                mul = (mul * 10) % n;
                val += mul;
                ret++;
			}

            Console.WriteLine(ret);
        }
    }
}