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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;

public class Main {
    public void main(Scanner sc) {
        int n = sc.nextInt();
        int m = sc.nextInt();
        char s[] = sc.next().toCharArray();
        char t[] = sc.next().toCharArray();
        sc.close();

        System.out.println(n < m ? check(n, m, s, t) : check(m, n, t, s));
    }

    private long check(int min, int max, char s[], char t[]) {
        int g = gcm(max, min);
        int minp = min / g;
        int maxp = max / g;
        for (int i = 0; i < g; i++) {
            if (s[i * minp] != t[i * maxp]) {
                return -1;
            }
        }

        return ((long) min) * maxp;
    }

    private int gcm(int a, int b) {
        return b == 0 ? a : gcm(b, a % b);
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        new Main().main(sc);
    }
}