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 proconio::{fastout, input};

#[fastout]
fn main() {
    input! {k: u32};
    let mut base = 7;
    let mut l = 1;
    let k: u32 = k;

    if k % 2 == 0 {
        println!("-1")
    } else {
        while k > base {
            base = base * 10 + 7;
            l += 1;
        }

        let mut m = base % k;

        let mut limit = 0;
        while m != 0 && limit < k {
            m = (m * 10 + 7) % k;
            limit += 1;
            l += 1;
        }

        if limit == k {
            println!("-1")
        } else {
            println!("{}", l)
        }
    }
}