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: Wrong Answer)

#include<bits/stdc++.h>
using namespace std;

int n;
int seven = 7;
int cnt = 1;

int main(){
	cin>>n;
	if(n%2==0){
		cout<<"-1"<<endl;
		return(0);
	}
	while(cnt<8){
		if(seven%n == 0){
			cout<<cnt<<endl;
			return(0);
		}
		seven = seven*10+7;
		cnt++;
	}
	int isp = 0;
	for(int i=2;i<n;i++){
		if(n%i==0)cout<<"-1"<<endl;
		return(0);
	}
	cout<<n-1<<endl;
	return(0);

}