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# (Mono-mcs 6.8.0.105) (judge verdict: Time Limit Exceeded)

using System;

class Program
{
    static void Main()
    {
        int K = int.Parse(Console.ReadLine());
        double seven = 0;
        int count = 0;

        if (K%2==0)
        {
            Console.WriteLine("-1");
        }
        else
        {
            for (int i = 0; ; i++)
            {
                seven += 7 * Math.Pow(10, i);
                count++;

                if (seven%K==0)
                {
                    Console.WriteLine(count);
                    break;
                }
            }
        }
    }
}