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>
#include<string>
#include<cmath>

using namespace std;

#define ll long long int
#define ld long double
#define loop(i, n) for (int i = 0; i < n; i++)
#define loops(i, s, n) for (int i = s; i < n; i++)
#define pb push_back
#define mp make_pair
#define all(v) v.begin(),v.end()
#define sz(x) ((int)(x).size())
#define fi first
#define se second
#define deb(x) cout<< #x << '=' << x <<endl
#define MOD 1000000007
const int N = 1e6 + 1;
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int k;
    cin>>k;
    if(k % 2 == 0 || k % 5 == 0){
        cout<<"-1\n";
        return 0;
    }
    int val = 7 % k;
    for(int i = 1; i < N; i++){
        if(val == 0){
            cout<<i;
            break;
        }
        val = (val*10 + 7) % k;
    }
}