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: Wrong Answer)

#include <iostream>
#include <string>
#include <vector>
#include <cstdlib>
#include <algorithm>
#include <set>
#include <stdio.h>
using namespace std;

int main() {
	int64_t k; cin >> k;
	int64_t s = 0; int64_t d = 1; int64_t i = 0;
	for (i = 0; i < 999999 ; i++) {
		s = s + (7 * d);
		if (s % k == 0)break;
		d = d * 10;
	}

	if (i > 999998) {
		cout << -1 << endl;
	}
	else {
		cout << i + 1 << endl;
	}
	return 0;
}