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 <iostream>

int main()
{
    int k;
    std::cin >> k;
    if (k % 2 == 0 || k % 5 == 0)
    {
        std::cout << -1 << '\n';
        return 0;
    }
    int cur = 0, pre = 1;
    for (int i = 1; i <= k; ++i)
    {
        cur += 7 * pre;
        cur %= k;
        if (cur == 0)
        {
            std::cout << i << '\n';
            return 0;
        }
        pre *= 10;
        pre %= k;
    }
}