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<cstring>
//#define Mfc_Tanzim
#define ll long long
#define ull unsigned long long
#define pb push_back
#define ff first
#define ss second
#define all(v) (v.begin(), v.end())
#define pi acos(-1.0)
#define FastRead ios_base::sync_with_stdio(0);cin.tie(0); cout.tie(0);
#define bug(a) cerr << #a << " : " << a << endl
#define pq dp
using namespace std;
const ll mx = 4444444444;

ll posx[] = {1,-1, 0, 0};
ll posy[] = {0, 0, 1,-1};

int main()
{
    FastRead

#ifdef Mfc_Tanzim
    freopen("input.txt","r", stdin);
    // freopen("output.txt","w", stdout);
#endif /// Mfc_Tanzim

    ll t=1, n, k, m, a, b, c, d;
    //cin >> t;

    while(t--){
              cin >> n;
              k = 0;
              for(ll i=1; i<1000000; i++){
                              k = (k*10+7)%n;
                              if(k){
                                       continue;
                              }
                              else{
                                       cout << i << endl;
                                       return 0;
                              }
              }
              cout << "-1" << endl;
    }
    return 0;
}