#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pb(...) emplace_back(__VA_ARGS__)
#define mp(a, b) make_pair(a, b)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define lscan(x) scanf("%I64d", &x)
#define lprint(x) printf("%I64d", x)
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define rep2(i, n) for (ll i = (ll)n - 1; i >= 0; i--)
#define REP(i, l, r) for (ll i = l; i < (r); i++)
#define REP2(i, l, r) for (ll i = (ll)r - 1; i >= (l); i--)
#define siz(x) (ll) x.size()
template <class T>
using rque = priority_queue<T, vector<T>, greater<T>>;

template <class T>
bool chmin(T &a, const T &b) {
    if (b < a) {
        a = b;
        return 1;
    }
    return 0;
}

template <class T>
bool chmax(T &a, const T &b) {
    if (b > a) {
        a = b;
        return 1;
    }
    return 0;
}

ll gcd(ll a, ll b) {
    if (a == 0)
        return b;
    if (b == 0)
        return a;
    ll cnt = a % b;
    while (cnt != 0) {
        a = b;
        b = cnt;
        cnt = a % b;
    }
    return b;
}

long long extGCD(long long a, long long b, long long &x, long long &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    long long d = extGCD(b, a % b, y, x);
    y -= a / b * x;
    return d;
}

struct UnionFind {
    vector<ll> data;
    int num;

    UnionFind(int sz) {
        data.assign(sz, -1);
        num = sz;
    }

    bool unite(int x, int y) {
        x = find(x), y = find(y);
        if (x == y)
            return (false);
        if (data[x] > data[y])
            swap(x, y);
        data[x] += data[y];
        data[y] = x;
        num--;
        return (true);
    }

    int find(int k) {
        if (data[k] < 0)
            return (k);
        return (data[k] = find(data[k]));
    }

    ll size(int k) {
        return (-data[find(k)]);
    }

    bool same(int x, int y) {
        return find(x) == find(y);
    }
};

template <int mod>
struct ModInt {
    int x;

    ModInt() : x(0) {
    }

    ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {
    }

    ModInt &operator+=(const ModInt &p) {
        if ((x += p.x) >= mod)
            x -= mod;
        return *this;
    }

    ModInt &operator-=(const ModInt &p) {
        if ((x += mod - p.x) >= mod)
            x -= mod;
        return *this;
    }

    ModInt &operator*=(const ModInt &p) {
        x = (int)(1LL * x * p.x % mod);
        return *this;
    }

    ModInt &operator/=(const ModInt &p) {
        *this *= p.inverse();
        return *this;
    }

    ModInt operator-() const {
        return ModInt(-x);
    }

    ModInt operator+(const ModInt &p) const {
        return ModInt(*this) += p;
    }

    ModInt operator-(const ModInt &p) const {
        return ModInt(*this) -= p;
    }

    ModInt &operator++() {
        return *this += ModInt(1);
    }

    ModInt operator++(int) {
        ModInt tmp = *this;
        ++*this;
        return tmp;
    }

    ModInt &operator--() {
        return *this -= ModInt(1);
    }

    ModInt operator--(int) {
        ModInt tmp = *this;
        --*this;
        return tmp;
    }

    ModInt operator*(const ModInt &p) const {
        return ModInt(*this) *= p;
    }

    ModInt operator/(const ModInt &p) const {
        return ModInt(*this) /= p;
    }

    bool operator==(const ModInt &p) const {
        return x == p.x;
    }

    bool operator!=(const ModInt &p) const {
        return x != p.x;
    }

    ModInt inverse() const {
        int a = x, b = mod, u = 1, v = 0, t;
        while (b > 0) {
            t = a / b;
            swap(a -= t * b, b);
            swap(u -= t * v, v);
        }
        return ModInt(u);
    }

    ModInt pow(int64_t n) const {
        ModInt ret(1), mul(x);
        while (n > 0) {
            if (n & 1)
                ret *= mul;
            mul *= mul;
            n >>= 1;
        }
        return ret;
    }

    friend ostream &operator<<(ostream &os, const ModInt &p) {
        return os << p.x;
    }

    friend istream &operator>>(istream &is, ModInt &a) {
        int64_t t;
        is >> t;
        a = ModInt<mod>(t);
        return (is);
    }

    static int get_mod() {
        return mod;
    }
};

ll mpow2(ll x, ll n, ll mod) {
    ll ans = 1;
    while (n != 0) {
        if (n & 1)
            ans = ans * x % mod;
        x = x * x % mod;
        n = n >> 1;
    }
    return ans;
}
ll modinv2(ll a, ll mod) {
    ll b = mod, u = 1, v = 0;
    while (b) {
        ll t = a / b;
        a -= t * b;
        swap(a, b);
        u -= t * v;
        swap(u, v);
    }
    u %= mod;
    if (u < 0)
        u += mod;
    return u;
}

// constexpr int mod = 1000000007;
constexpr int mod = 998244353;
// constexpr int mod = 31607;
using mint = ModInt<mod>;

mint mpow(mint x, ll n) {
    mint ans = 1;
    while (n != 0) {
        if (n & 1)
            ans *= x;
        x *= x;
        n = n >> 1;
    }
    return ans;
}

// ----- library -------
vector<int> manacher(const string &s) {
    vector<int> radius(s.size());
    int i = 0, j = 0;
    while (i < s.size()) {
        while (i - j >= 0 && i + j < s.size() && s[i - j] == s[i + j]) {
            ++j;
        }
        radius[i] = j;
        int k = 1;
        while (i - k >= 0 && i + k < s.size() && k + radius[i - k] < j) {
            radius[i + k] = radius[i - k];
            ++k;
        }
        i += k;
        j -= k;
    }
    return radius;
}
// ----- library -------

int main() {
    ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    cout << fixed << setprecision(15);

    int n, m;
    string s, t;
    cin >> n >> m >> s >> t;
    vector<int> fs(26, 0), ft(26, 0);
    rep(i, n) fs[s[i] - 'a'] = 1;
    rep(i, m) ft[t[i] - 'a'] = 1;
    rep(i, 26) {
        if (ft[i] && !fs[i]) {
            cout << -1 << endl;
            return 0;
        }
    }
    if (m % 2) {
        cout << -1 << endl;
        return 0;
    }
    rep(i, m) {
        if (t[i] != t[m - 1 - i]) {
            cout << -1 << endl;
            return 0;
        }
    }
    int pre = 0, suf = 0;
    while (pre < min(n, m)) {
        if (s[pre] == t[pre])
            pre++;
        else
            break;
    }
    while (suf < min(n, m)) {
        if (s[n - 1 - suf] == t[m - 1 - suf])
            suf++;
        else
            break;
    }
    string ct;
    rep(i, m) ct += '$', ct += t[i];
    auto l = manacher(ct);
    vector<int> mi(m * 3, 1e9);
    rep(i, m) chmin(mi[(l[i * 2] - 1) / 2 + i], (int)i);
    rep2(i, m * 3 - 1) chmin(mi[i], mi[i + 1]);
    int nlen = m / 2, ans = 1;
    int tar = max(pre, suf);
    while (nlen > tar) {
        ans++;
        if (nlen <= mi[nlen]) {
            cout << -1 << endl;
            return 0;
        }
        nlen = mi[nlen];
    }
    cout << ans << endl;
}