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: Wrong Answer)

using System;
class program{
  static void Main(){
    int k = int.Parse(Console.ReadLine());
    if(k%2 == 0){
      Console.Write(-1);
      Environment.Exit(0);
    }else if(k%5 == 0){
      Console.Write(-1);
      Environment.Exit(0);
    }else if(k%7 == 0){
      k = k/7;
    }
    int ans = 1,i=10;
    
    while(i != 1){
      i *= 10;
      i %= k;
      ans++;
    }
    Console.Write(ans);
  }
}