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 (GCC 9.2.1) (judge verdict: Accepted)

#include<stdio.h>

int main(){
  int i;
  int K;
  int a = 7 % K;

  
  scanf("%d" ,&K);
  
  if(K == 7){
    
    a = a % K;
    
  }
  
  if(K % 2 != 0 && K % 5 != 0){
  
    for(i = 1; i < K; i++){

       if(a == 0)break;
       a = (a * 10 + 7) % K;     
      
    }
    
  }else{
    
    i = -1;
  
  }
  
  printf("%d\n" ,i);
  
  return 0;
  
}