Problem:
Score : 300 points

Problem Statement

You are given a string S of length N and another string T of length M.
These strings consist of lowercase English letters.

A string X is called a good string when the following conditions are all met:

Let L be the length of X. L is divisible by both N and M.

Concatenating the 1-st, (\frac{L}{N}+1)-th, (2 \times \frac{L}{N}+1)-th, ..., ((N-1)\times\frac{L}{N}+1)-th characters of X, without changing the order, results in S.

Concatenating the 1-st, (\frac{L}{M}+1)-th, (2 \times \frac{L}{M}+1)-th, ..., ((M-1)\times\frac{L}{M}+1)-th characters of X, without changing the order, results in T.

Determine if there exists a good string. If it exists, find the length of the shortest such string.

Constraints

1 \leq N,M \leq 10^5

S and T consist of lowercase English letters.

|S|=N

|T|=M

Input

Input is given from Standard Input in the following format:

N M
S
T

Output

If a good string does not exist, print -1; if it exists, print the length of the shortest such string.

Sample Input 1

3 2
acp
ae

Sample Output 1

6

For example, the string accept is a good string.
There is no good string shorter than this, so the answer is 6.

Sample Input 2

6 3
abcdef
abc

Sample Output 2

-1

Sample Input 3

15 9
dnsusrayukuaiia
dujrunuma

Sample Output 3

45

Solution in Rust (1.42.0) (judge verdict: Wrong Answer)

// -*- coding:utf-8-unix -*-

use proconio::input;

fn gcd(a: u64, b: u64) -> u64 {
    if b == 0 {
        a
    } else {
        gcd(b, a % b)
    }
}

fn lcm(a: u64, b: u64) -> u64 {
    a / gcd(a, b) * b
}

fn main() {
    input! {
        n: u64,
        m: u64,
        s: String,
        t: String,
    }

    let vs: Vec<char> = s.chars().collect();
    let vt: Vec<char> = t.chars().collect();
    if vs[0] != vt[0] {
        println!("-1");
        return;
    }

    if m > n {
        // swap
        if m % n == 0 {
            let mut i = 0;
            let r = m / n;
            while i < m {
                if vt[i as usize] != vs[(i / r) as usize] {
                    println!("-1");
                    return;
                }
                i += r;
            }
        }
    } else {
        // n >= m
        if n % m == 0 {
            let mut i = 0;
            let r = n / m;
            // println!("{:?}, {:?}", vs, vt);
            while i < n {
                // println!("{} {}", vs[i as usize], vt[(i / r) as usize]);
                if vs[i as usize] != vt[(i / r) as usize] {
                    println!("-1");
                    return;
                }
                i += r;
            }
        }
    }

    println!("{}", lcm(n, m));
}