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: Time Limit Exceeded)

#include	<stdio.h>

int		main(void)
{
	int k, count, sum, t;
	int i;
	count = sum = t = 0;

	scanf("%d", &k);

	for (i = 7; t != 1; i+=7) {
		if (k % 2 == 0) {
			count = -1;
			break;
		}

		if (i < k) {
			count++;
			i = i * 10;
		}
		else {
			sum = i % k;
			if (sum == 0) {
				t = 1;
				count++;
			}
			else {
				count++;
				i *= 10;
			}
		}
	}

	printf("%d\n", count);

	return 0;
}