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<bits/stdc++.h>

using namespace std;

#define endl "\n"
#define lln long long int
#define IOS ios::sync_with_stdio(0);cin.tie(0);

int freq[1000001];

int main(){
    IOS;
    int k;
    cin >> k;
    if(!(k%2)){
        cout << -1 << endl;
        return 0;
    }
    int a = 7;
    for(int i=0;;i++){
        if(a % k == 0){
            cout << i + 1 << endl;
            return 0;
        }
        else{
            int b = a % k;
            if(freq[b]) break;
            freq[b]++;
            a = (a * 10 + 7) % k;
        }
    }
    cout << -1 << endl;
    return 0;
}