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: Accepted)

#include<iostream>
#include<vector>
#include<set>
#include<unordered_set>
#include<string>
#include<stack>
#include<algorithm>

using namespace std;

void slove(){
    int k;
    int cnt = 1;
    
    cin>>k;
    if(k%2 == 0 || k%5 == 0){
        cout<<-1<<endl;
        return;
    }

    int res = 7%k;
    while(res){
        res = (res*10+7)%k;
        cnt++;
    }

    cout<<cnt<<endl;
}

int main(){
    slove();
}