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 c
{
    class Program
    {
        static void Main(string[] args)
        {
            var k = int.Parse(Console.ReadLine());

            long[] a = new long[k];
            a[0] = 7;
            for (int i = 1; i < k; ++i) {
                a[i] = (a[i - 1] * 10 + 7) % k;
            }
            for (int i = 0; i < k; ++i) {
                if (a[i] % k == 0) {
                    Console.WriteLine(i + 1);
                    return;
                }
            }
            Console.WriteLine(-1);
            return;
        }
    }
}