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>
using namespace std;

int main() {
    int k;
    cin >> k;
    
    if (k%2==0 || k%5==0 || k%17==0 || k%19==0 || k%23==0 || k%29==0 || k%31==0) {
        cout << -1 << endl;
        return 0;
    }
    else {
        long long s = 7, o = 10, ans = 1;
        while (1) {
            if (s%k == 0) {
                cout << ans << endl;
                break;
            }
            s += o*7;
            o *= 10;
            ans++;
        }
    }
}