//
//
//  Codeforces: ganariya
//  AtCoder: ganariya2525
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//  MMMMMMMMMMMMMMMMNOtXtdMMMMMNxzZMMNmgHyVVVVVW%..?/(.,(x-..........(HfffpffpffpffWHHffpffppfVyyWMMMMMM


//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>

using namespace std;

#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v){std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}

using LL = long long;

//~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~

LL gcd(LL x, LL y) {
    return y == 0 ? x : gcd(y, x % y);
}

LL lcm(LL x, LL y) {
    return x / gcd(x, y) * y;
}

int main() {

    LL N, M;
    cin >> N >> M;

    string S, T;
    cin >> S >> T;

    LL L = lcm(N, M);

    map<LL, char> mp;
    for (int i = 0; i < S.size(); i++) {
        mp[i * (L / N)] = S[i];
    }
    for (int i = 0; i < T.size(); i++) {
        if (mp.count(i * (L / M)) && mp[i * (L / M)] != T[i]) {
            cout << -1;
            return 0;
        }
        mp[i * (L / M)] = T[i];
    }

    cout << L;

    return 0;
}










































Measured memory_bytes: 10880