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;


main()
{
    int n;
    cin>>n;
    int r=7%n;
    if(n%2==0 or n%5==0)
    {
        cout<<-1<<endl;
        return 0;
    }

    int prevr=0, ans=1;
    bool t=true;
    while(r)
    {
        int a=r*10+7;
        ans++;
        r=a%n;
        if(a/n)
        {
            if(r==prevr)
            {
                t=false;
                break;
            }else
            {
                prevr=r;
            }
        }
    }
    if(t)
        cout<<ans<<endl;
    else
        cout<<-1<<endl;
}