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 Rust (1.42.0) (judge verdict: Accepted)

use std::collections;
use whiteread::parse_line;

fn main() {
    let k: usize = parse_line().unwrap();
    let mut s = collections::HashSet::new();
    let mut x = 7 % k;
    let mut i = 1;
    for i in 1.. {
        if s.contains(&x) {
            println!("{}", -1);
            return;
        }
        if x == 0 {
            println!("{}", i);
            return;
        }
        s.insert(x);
        x = (x * 10 + 7) % k
    }
}