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;

namespace AtCoder
{
    internal static class ProgramC
    {
        private class Solver
        {
            public void Solve()
            {
                int k = int.Parse(Console.ReadLine());

                long test = 0;
                long memo = 0;
                for (int i = 0;; ++i)
                {
                    test = test*10 + 7;
                    if (test < k) continue;

                    test = test%k;
                    if (test == 0)
                    {
                        Console.WriteLine(i + 1);
                        return;
                    }
                    if (memo == 0)
                    {
                        memo = test;
                        continue;
                    }
                    if (memo == test)
                    {
                        Console.WriteLine(-1);
                        return;
                    }
                }
            }

            private static void Main(string[] args)
            {
                new Solver().Solve();
            }
        }
    }
}