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>
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define REP(i,a,b) for(int i=int(a);i<(int)b;++i)
#define Graph vector<vecctor<ll>>
#define pairs vector<pair<ll,ll>>
#define pb push_back



using namespace std;
using ll = long long;
const ll INF = 1e18;
const double PI = acos(-1);

template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}



int main(void)
{
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);
    ll k,ans=0,num=7;
    cin >> k;
    vector<ll> a;
    ll index = 1;
    bool flag = true;
    if(num%k == 0){
        cout << 1 << endl;
        return 0;
    }
    rep(i,k){
        if(num == 0){
            flag = false;
            break;
        }
        a.pb(num);
        num = (num*10+7)%k;
        index++;
    }
    if(flag){
        cout << -1 << endl;
    }else{
        cout << index << endl;
    }
    return 0;

}