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 int long long
#define ll long long
#define FOR(i,a,b) for(int i=(a); i<=(b); i++)
#define rep(i,n) for(int i = 0; i < (n); i++)
#define loop(k) rep(author_is_masa,k)
#define chmin(a,b) if((a)>(b)) (a)=(b);
#define chmax(a,b) if((a)<(b)) (a)=(b);
#define vi vector<int>
#define pii pair<int,int>
#define all(v) (v).begin(),(v).end()
#define allr(v) (v).rbegin(),(v).rend()
#define pb push_back
#define pf push_front
int gcd(int a,int b){/*a>=0,b>=0,￢(a=b=0)*/
  while(min(a,b)>0){if(a<b)swap(a,b);a%=b;}return max(a,b);
}
int dx[]={1,0,-1,0,1,-1,-1,1};
int dy[]={0,1,0,-1,1,1,-1,-1};
const int MOD = 1e9+7;
const long long INF = 1e18;

/*--------------------------------------------------------------------*/

signed main(){
	int k;cin>>k;
	if(k%2==0){
		cout<<-1<<endl;
		return 0;
	}
	
	int now=0;
	for(int i=1;i<1e7;i++){
		now=(now*10+7)%k;
		if(now==0){
			cout<<i<<endl;
			return 0;
		}
	}
	cout<<-1<<endl;
	

	return 0;
}