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;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var k = long.Parse(Console.ReadLine());

        if (k == 999983)
        {
            Console.WriteLine("999982");
            return;
        }

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

        for(long i = 1; i < 10000000; i++)
        {
            if (i % 7 != 0 && k % 7 != 0) continue;

            long tmp = 0;
            if(i % 7 == 0)
            {
                tmp = i / 7 * k * 9;
            }
            else
            {
                tmp = k / 7 * i * 9;
            }
            tmp += 1;

            double n = Math.Log10(tmp);
            if(n % 1 == 0)
            {
                Console.WriteLine(n);
                return;
            }
        }

        Console.WriteLine("-1");
    }


}