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 static System.Console;
using System.Collections.Generic;

namespace abc174
{
    class Program
    {
        static void Main()
        {
            var k = int.Parse(ReadLine());
            var set = new HashSet<int>();
            var num = 7;
            var count = 1;
            while (true)
            {
                num = num % k;
                if (num == 0)
                {
                    WriteLine(count);
                    return;
                }
                if (set.Contains(num))
                {
                    WriteLine(-1);
                    return;
                }
                set.Add(num);
                num = num * 10 + 7;
                ++count;
            }
        }
    }
}