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: Accepted)

using System;
using System.Linq;

namespace Program
{
    class C
    {
        static void Main(string[] args)
        {
            var K = int.Parse(Console.ReadLine());
            int ret = -1;
            if (K % 7 == 0)
            {
                K /= 7;
            }

            if (K % 5 == 0 || K % 2 == 0)
            {
                ret = -1;
            }
            else
            {
                K *= 9;
                int res = -1;
                for (int i = 0; i < 1000000; i++)
                {
                    if (res == -1)
                    {
                        if (Power(10, i) >= K)
                        {
                            res = Power(10, i) % K;
                        }
                    }
                    else if (res == 1)
                    {
                        ret = i-2;
                        break;
                    }
                    else
                    {
                        res = (10 * res) % K;
                    }

                }
            }
            Console.WriteLine(ret);
        }
        public static int Power(int x, int n)
        {
            int ret = 1;

            for (int i = 0; i < n - 1; i++)
            {
                ret *= x;
            }
            return ret;

        }
    }
}