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>
#include <vector>

#define rep(x, n) for(int i = x; i < (n); ++i)

using namespace std;
using ll = long long;

int main(void)
{
  ll k;
  cin >> k;

  ll res = 0;
  ll n = 7;
  if (k % 2 == 0) {
    res = -1;
  } else {
    while (res < 100000001) {
      ++res;
      if (n % k == 0) break;
      n = n * 10 + 7;
    }
  }
  cout << (res == 100000001 ? k - 1 : res) << endl;
}