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.Linq;
using System.Collections.Generic;

namespace AtCoder0802
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            long K = long.Parse(Console.ReadLine());
            long seven = 7;
            long ans = 1;
            long lseven;

            if (K % 2 != 0)
            {
                for (var i = 1; i < 1000000; i++)
                {
                    if(seven % 7 != 0) { continue; }
                        if (seven % K == 0)
                        {
                            break;
                        }
                        else
                        {
                            seven = seven*10 + 7;
                            ans++;
                        }
                    //}
                }
            }
            if (K % 2 == 0)
            {
                Console.WriteLine("-1");
            }
            else
            {
                var test = seven % K;
                Console.WriteLine(ans);
            }
        }
    }
}