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 <cmath>
#include <climits>
using namespace std;

const long max_index = 1e6;
int main() {
    long k, prod = 7, index=1;
    cin>>k;
    if(!(k%2)) {
        cout<<"-1";
    } else {
        while(index<max_index) {
            if(!(prod%k)) {
                break;
            }
            prod = prod * 10 + 7;
            ++index;
        }
        if(index==max_index)
            cout<<"-1";
        else
            cout<<index;
    }
    return 0;
}