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 <bits/stdc++.h>

#define FAST ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
using namespace std;
using ll = long long;

int len(int n) {
    if (n % 2 == 0) return -1;

    int i, k = 0;

    for (i = 1; i <= 1000000; ++i) {
        k = (k*10+1) % n;
        if (!k) return i;
    }
    if (i) return -1;
}

int main() {
    FAST;

    int n;
    cin >> n;
    cout << len(n);

    return 0;
}