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)

//
// Created by Sakurayama Kazuhiro on 2020/08/02.
//


#include<iostream>
#include <math.h>       /* pow */
#include <set>

using namespace std;
int main(){
    unsigned int K;
    cin >> K;

    if (K % 2 == 0 || K % 5 == 0 ) cout << "-1" << endl;

    unsigned int cnt = 0;
    unsigned int a = 7;

    std::set<unsigned long long> myset;
    bool solved = false;

    while (true){
        unsigned int aa = a % K;
        if (aa == 0) {
            solved = true;
            break;
        }
        if (myset.find(aa) != myset.end()) break;
        myset.insert(aa);
        cnt++;
        a = (unsigned int) aa*10+7;
    }

    if (solved) {
        cout << cnt+1 << endl;
    } else {
        cout << -1 << endl;
    }
    return 0;
}