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;
 
typedef long long ll;
typedef long double ld;
 
typedef pair<ll, pair<ll, ll> > p;
 
#define MOD 1000000007
 
int main() {
	cin.tie(0);
    ios::sync_with_stdio(false);

    ll k;
    cin >> k;

    ll x = 7LL;

    ll ans = 1LL;

    for(int i = 0; i < pow(10, 6); i++) {
        if(x >= k) {
            x %= k;
            if(x == 0LL) {
                break;
            }
        }
        x *= 10LL;
        x += 7LL;
        ans++;
    }

    if(x == 0LL) {
        cout << ans << endl;
    } else {
        cout << -1 << endl;
    }
}