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

namespace ConsoleApp3
{
    public class C
    {
        static void Main(string[] args)
        {
            var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            Console.SetOut(sw);
            Solve();
            Console.Out.Flush();
        }

        public static void Solve()
        {
            var K = int.Parse(Console.ReadLine());
            if (K % 2 == 0 )
            {
                Console.WriteLine(-1);
                return;
            }
            //var n = 7L;
            //var answer = 1L;
            //while (n % K != 0)
            //{
            //    n *= 10;
            //    n += 7;
            //    n %= K;
            //    answer++;
            //}

            //好きな数字である７を初期化
            int num = 7;
            //何項目を初期化
            int count = 0;

            for (int i = 1; i <= K; i++)
            {
                //ｋの倍数かどうかを判断
                if (num % K == 0)
                {
                    count = i;
                    break;
                }
                //乗余はｋの倍数かどうかを判断
                num = (num * 10 + 7) % K;
            }
            //何項目を出力
            Console.WriteLine(count);

            //Console.WriteLine(answer);
        }
    }
}