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 fastio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define endl "\n"

using namespace std;
using ll = long long;

void solve()
{
  int k;
  cin >> k;

  if (k % 2 == 0)
  {
    cout << -1;
    return;
  }

  ll num = 0;
  for(int i = 1; i <= k; ++i)
  {
    num = num * 10 + 7;
    num %= k;
    if (num == 0)
    {
      cout << i;
      return;
    }
  }  
  cout<< -1;
}

int main()
{
  fastio;
  solve();

  return 0;
}