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 ll long long
#define FOR(i, a, b) for (int i=a; i<(b); i++)

ll k;

void solve(){
    if (k%2 == 0 || k%5 == 0){
        cout << "-1\n";
        return;
    }
    ll ans = 1;
    ll cur = 7 % k;
    while (cur != 0){
        cur = cur * 10 + 7;
        cur %= k;
        ans++;
    }
    cout << ans << "\n";
}


int main(){
   	ios::sync_with_stdio(false);
	cin.tie(0);
	cin >> k;
	solve();
	return 0;
}