void main() {
	auto ip = readAs!(ulong[]), N = ip[0], M = ip[1];
	auto S = rs;
	auto T = rs;
	auto L = (N*M) / gcd(N, M);
	byte[] C = new byte[](L+1);
	C[] = byte.max;

	//writeln((M-1) * L / M + 1);
	foreach(i; iota(0, (N-1) * L / N + 1, L / N)) {
		auto j = i / (L / N);
		C[i] = (S[j] - 'a').to!byte;
		//i.writeln;
	}
	//C.writeln;
	foreach(i; iota(0, (M-1) * L / M + 1, L / M)) {
		//i.writeln;
		auto j = i / (L / M);
		if(C[i] == byte.max) {
			C[i] = (T[j] - 'a').to!byte;
			continue;
		}
		//writeln(C[i], T[j] - 'a');
		//writeln(0);
		if(C[i] != T[j] - 'a') {
			writeln(-1);
			return;
		}
		C[i] = (T[j] - 'a').to!byte;
	}
	writeln(L);
}

// ===================================

import std.stdio;
import std.string;
import std.functional;
import std.algorithm;
import std.range;
import std.traits;
import std.math;
import std.container;
import std.bigint;
import std.numeric;
import std.conv;
import std.typecons;
import std.uni;
import std.ascii;
import std.bitmanip;
import core.bitop;

T readAs(T)() if (isBasicType!T) {
	return readln.chomp.to!T;
}
T readAs(T)() if (isArray!T) {
	return readln.split.to!T;
}

T[][] readMatrix(T)(uint height, uint width) if (!isSomeChar!T) {
	auto res = new T[][](height, width);
	foreach(i; 0..height) {
		res[i] = readAs!(T[]);
	}
	return res;
}

T[][] readMatrix(T)(uint height, uint width) if (isSomeChar!T) {
	auto res = new T[][](height, width);
	foreach(i; 0..height) {
		auto s = rs;
		foreach(j; 0..width) res[i][j] = s[j].to!T;
	}
	return res;
}

int ri() {
	return readAs!int;
}

double rd() {
	return readAs!double;
}

string rs() {
	return readln.chomp;
}
