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 <iostream>
using namespace std;

int main(){
  int K;
  cin >> K;
  if(K%7==0){
    K=K/7;

  }
  if(10%K==0){
    cout << -1 << endl;
    return 0;
  }
  int S=1;
  int T=1;
  for(int i=0; i<1000000; i++){
    if(S%K==0){
      cout << i+1 << endl;
      break;
    }
      S=S+10*T;
      S=S%K;
      T=10*T%K;
  }
  return 0;
  }