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: Time Limit Exceeded)

#include <iostream>
#include <vector>
#include <string>
#include <math.h>
#include <algorithm>
#include <set>
#include <iomanip>
#include <stdio.h>
#include <sstream>
#include <bits/stdc++.h>
using namespace std;
int main(void){
  long long int k;
  cin>>k;
  if(k%2==0){
    cout<<-1<<endl;
  }else{
    long long int now=7;
    long long int count=1;
    while(true){
      if(now%k==0){
        cout<<count<<endl;
        break;
      }else{
        now*=10;
        now+=7;
        now%=k;
        count++;
      }
    }
  }
}