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 System.Linq;
using System.Collections.Generic;
using System.Numerics;

namespace ABC174
{
    class Program
    {
        static void Main(string[] args)
        {
            //var NK = Console.ReadLine().Split().Select(long.Parse).ToArray();
            //var p = Console.ReadLine().Split().Select(long.Parse).OrderBy(x => x).ToArray();
            // var CA = Enumerable.Repeat(0, (int)N).Select(_ => Console.ReadLine().Split().Select(long.Parse).ToArray()).ToArray();
            var K = long.Parse(Console.ReadLine());
            var x = 7L;
            for (long i = 0; i <= 10000000; i++)
            {
                if (x % K == 0) { Console.WriteLine(i + 1); return; }
                x = (10 * x + 7) % K;
            }
            Console.WriteLine(-1);
        }
    }
}