#include <iostream>
#include <fstream>
#include <cmath>  
#include <cstdlib>
#include <ctime>
#include <algorithm>
#include <numeric>
#include <functional>
#include <string> 
#include <vector>
#include <bitset>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <deque>

using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;

#define REP(i,n) for(int i = 0; i < int(n); i++)
#define REREP(i,n) for(int i = int(n - 1); i >= 0; i--)
#define FOR(i, m, n) for(int i = int(m);i < int(n);i++)
#define REFOR(i, m, n) for(int i = int(n - 1);i >= int(m);i--)
#define ALL(obj) (obj).begin(),(obj).end()

#define VI vector<int>
#define VVI vector<vector<int>>
#define VVVI vector<vector<vector<int>>>
#define VD vector<double>
#define VVD vector<vector<double>>
#define VVVD vector<vector<vector<double>>>
#define VL vector<ll>
#define VVL vector<vector<ll>>
#define VVVL vector<vector<vector<ll>>>
#define VB vector<bool>
#define VVB vector<vector<bool>>
#define VVVB vector<vector<vector<bool>>>
#define VS vector<string>
#define VVS vector<vector<string>>
#define VVVS vector<vector<vector<string>>>

#define PII pair<int,int>
#define PDD pair<double,double>
#define PLL pair<ll,ll>
#define VPII vector<pair<int,int>>
#define VVPII vector<vector<pair<int,int>>>
#define VVVPII vector<vector<vector<pair<int,int>>>>
#define VPLL vector<pair<ll,ll>>
#define VVPLL vector<vector<pair<ll,ll>>>
#define VVVPLL vector<vector<vector<pair<ll,ll>>>>

#define SUMI(obj) accumulate((obj).begin(), (obj).end(), 0)
#define SUMD(obj) accumulate((obj).begin(), (obj).end(), 0.)
#define SUML(obj) accumulate((obj).begin(), (obj).end(), 0LL)
#define SORT(obj) sort((obj).begin(), (obj).end()) // 1,2,3,4,5...
#define RESORT(obj) sort((obj).begin(), (obj).end(), greater<int>()) // 5,4,3,2,1...
#define UB(obj,n) upper_bound((obj).begin(), (obj).end(), n) //itr > n
#define LB(obj,n) lower_bound((obj).begin(), (obj).end(), n) //itr>= n

const ll HMOD = (ll)1e9 + 7;
const ll LMOD = 998244353;
const ll HINF = (ll)1e18;
const ll LINF = (ll)1e9;
const long double PI = 3.1415926535897932384626433;
const long double EPS = 1e-10;

template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {
	o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;
}

template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {
	o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;
}

template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {
	o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;
}

template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {
	o << "{" << obj.first << ", " << obj.second << "}"; return o;
}

template <template <class tmp>  class T, class U> ostream &operator<<(ostream &o, const T<U> &obj) {
	o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr)o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;
}

void print(void) {
	cout << endl;
}

template <class Head> void print(Head&& head) {
	cout << head;
	print();
}

template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {
	cout << head << " ";
	print(forward<Tail>(tail)...);
}


void YN(bool flag) {
	cout << ((flag) ? "YES" : "NO") << endl;
}

void Yn(bool flag) {
	cout << ((flag) ? "Yes" : "No") << endl;
}

void yn(bool flag) {
	cout << ((flag) ? "yes" : "no") << endl;
}

//Greatest Common Divisor
long long GCD(long long a, long long b) {
	return ((b == 0) ? a : GCD(b, a % b));
}

//Least Common Multiple
long long LCM(long long a, long long b) {
	return ((a*b == 0) ? 0 : (a / GCD(a, b)*b));
}

int main() {
	ll N, M; cin >> N >> M;
	string S, T; cin >> S >> T;

	ll L = LCM(N, M);
	map<ll, int> mp1,mp2;
	REP(i, N) mp1[(L / N)*i] = S[i] - 'a';
	REP(i, M) mp2[(L / M)*i] = T[i] - 'a';

	REP(i,N) {
		if (mp2[(L / N)*i] == 0) continue;
		if (mp1[(L / N)*i] != mp2[(L / N)*i]){
			cout << -1 << endl;
			return 0;
		}
	}
		
	cout << L << endl;
	return 0;
}



Measured memory_bytes: 15616