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;

namespace Contest174
{
    class Program
    {
        static void Main()
        {            
int K = int.Parse(Console.ReadLine());
            double n;
            double m = 0;
            int output;
            for (int i = 1; i < 1000000; i++)
            {
                if (K % 2 == 0)
                {
                    Console.WriteLine("-1");
                    break;
                }
                n = Math.Pow(10, i);
                m = n / 10 * 7 + m;
                output = (int)m;
                if(output % K == 0)
                {
                    Console.WriteLine(i);
                    break;
                }
            }
        }
    }
}