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 (Clang 10.0.0) (judge verdict: Wrong Answer)

#include <stdio.h>

int main()
{
  int k;
  scanf("%d", &k);
  
  if (k % 7 != 0)
  {
    printf("-1");
	return 0;
  }
  
  k /= 7;
  
  int n = 1, i = 1;
  while (n <= k)
  {
	if (k % n == 0)
	{
	  printf("%d", i);
	  return 0;
	}
	else
	{
	  n *= 10;
	  n++;
	  i++;
	}
  }
  printf("-1");
  
  return 0;
}