#include <bits/stdc++.h>
using namespace std;

using ull = unsigned long long;
using ll = long long;

#define REP(i, n) for (int i = 0; i < n; i++)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define pb push_back
#define fill(x, y) memset(x, y, sizeof(x))
#define even(x) x % 2 == 0
#define odd(x) x % 2 != 0
#define all(x) x.begin(), x.end()
#define pcnt __builtin_popcount
#define buli(x) __builtin_popcountll(x)
#define F first
#define S second
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());
#define IN1(x) ll x; cin >> x;

// these functions return the position of result of Binary Search.
#define LB(s, t, x) (int) (lower_bound(s, t, x) - s)
#define UB(s, t, x) (int) (upper_bound(s, t, x) - s)

ll qp(ll a, ll b, int mo) { ll ans = 1; do { if (b & 1) ans = 1ll * ans * a % mo; a = 1ll * a * a % mo; } while (b >>= 1); return ans; }
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { ll temp = gcd(a, b); return temp ? (a / temp * b) : 0; }
int mDays[] = { 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
int dx8[] = { 1, -1, 0, 0, 1, 1, -1, -1 }, dy8[] = { 0, 0, -1, 1, -1, 1, -1, 1 };

template <typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts>
auto make_v(size_t a, size_t b, Ts... ts) { return vector<decltype(make_v<T>(b, ts...))>(a, make_v<T>(b, ts...)); }
template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type
fill_v(T &t, const V &v) { t = v; }
template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type
fill_v(T &t, const V &v) { for (auto &e : t) fill_v(e, v); }
// auto dp = make_v<int>(4, h, w);
// fill_v(dp, 0);

const ll INF_LL = (1ll << 60);
const int INF_INT = (int)1e9;
const ll MOD_CONST = (ll)(1e9 + 7);

template <typename T>
vector<T> pows(int b, int n) {
    vector<T> ret;
    T x = 1;
    while (ret.size() < n) {
        ret.push_back(x);
        x *= b;
    }
    return ret;
}
// find string "from" in "str", and replace them to "to"
void strReplace(std::string& str, const std::string& from, const std::string& to) {
	std::string::size_type pos = 0;
	while (pos = str.find(from, pos), pos != std::string::npos) {
		str.replace(pos, from.length(), to);
		pos += to.length();
	}
}
template <class T>
bool chmax(T &a, const T &b) { if (a < b) { a = b; return 1; } return 0; }
template <class T>
bool chmin(T &a, const T &b) { if (b < a) { a = b; return 1; } return 0; }

inline tuple<ll, ll> rotate45(tuple<ll, ll> point) { ll x = get<0>(point), y = get<1>(point); return tuple<ll, ll>(x + y, x - y); }
inline bool rangeCheck2D(int nx, int ny, int Width, int Height) { return nx >= 0 and nx < Width and ny >= 0 and ny < Height; }

template <typename T>
vector<T> INPA(ll n) {
	vector<T> x;
	REP(i, n) {
		T tmp; cin >> tmp;
		x.push_back(tmp);
	}
	return move(x);
}
template <typename T>
void out(T o) { cout << o << endl; }
template <typename T>
void out(vector<T> &out) { REP(i, (int)out.size()) { cout << out[i]; if (i == (int)out.size() - 1) cout << endl; else cout << " "; } }
template <typename T>
void out(vector<vector<T>> o) { REP(i, o.size()) out(o[i]); }
void YesNo(bool f) { cout << (f?"Yes":"No") << endl; }
void YESNO(bool f) { cout << (f?"YES":"NO") << endl; }


int main(void)
{
    cin.tie(0); ios::sync_with_stdio(0); cout << fixed << setprecision(20);

    IN1(N); IN1(M);
	string s1, s2; cin >> s1 >> s2;
	int id1 = 0, id2 = 0;
	ll num = lcm(N, M);
	ll n1 = num / N, n2 = num / M;
	int idx1 = 0, idx2 = 0;
	while (true) {
		if (idx1 >= N and idx2 >= M) break;
		if (id1 == id2) {
			if (s1[idx1] != s2[idx2]) {
				out(-1);
				return 0;
			}
			idx1++, idx2++;
			id1 += n1;
			id2 += n2;
		} else if (id1 > id2) {
			id2 += n2;
			idx2++;
		} else {
			id1 += n1;
			idx1++;
		}
	}
	out(num);

    return 0;
}

Measured memory_bytes: 640