#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < n; i++)
#define rep2(i, x, n) for (int i = x; i <= n; i++)
#define rep3(i, x, n) for (int i = x; i >= n; i--)
#define each(e, v) for (auto &e : v)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;

template <typename T>
bool chmax(T &x, const T &y) {
    return (x < y) ? (x = y, true) : false;
}

template <typename T>
bool chmin(T &x, const T &y) {
    return (x > y) ? (x = y, true) : false;
}

template <typename T>
int flg(T x, int i) {
    return (x >> i) & 1;
}

template <typename T>
void print(const vector<T> &v, T x = 0) {
    int n = v.size();
    for (int i = 0; i < n; i++) cout << v[i] + x << (i == n - 1 ? '\n' : ' ');
}

template <typename T>
void printn(const vector<T> &v, T x = 0) {
    int n = v.size();
    for (int i = 0; i < n; i++) cout << v[i] + x << '\n';
}

template <typename T>
int lb(const vector<T> &v, T x) {
    return lower_bound(begin(v), end(v), x) - begin(v);
}

template <typename T>
int ub(const vector<T> &v, T x) {
    return upper_bound(begin(v), end(v), x) - begin(v);
}

template <typename T>
void rearrange(vector<T> &v) {
    sort(begin(v), end(v));
    v.erase(unique(begin(v), end(v)), end(v));
}

template <typename T>
vector<int> id_sort(const vector<T> &v, bool greater = false) {
    int n = v.size();
    vector<int> ret(n);
    iota(begin(ret), end(ret), 0);
    sort(begin(ret), end(ret), [&](int i, int j) { return greater ? v[i] > v[j] : v[i] < v[j]; });
    return ret;
}

struct io_setup {
    io_setup() {
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout << fixed << setprecision(15);
    }
} io_setup;

const int inf = (1 << 30) - 1;
const ll INF = (1LL << 60) - 1;
const int MOD = 1000000007;
// const int MOD = 998244353;

template <typename Monoid>
struct Segment_Tree {
    using F = function<Monoid(Monoid, Monoid)>;
    int n;
    vector<Monoid> seg;
    const F f;
    const Monoid e1;

    // f(f(a,b),c) = f(a,f(b,c)), f(e1,a) = f(a,e1) = a

    Segment_Tree(const vector<Monoid> &v, const F &f, const Monoid &e1) : f(f), e1(e1) {
        int m = v.size();
        n = 1;
        while (n < m) n <<= 1;
        seg.assign(2 * n, e1);
        copy(begin(v), end(v), seg.begin() + n);
        for (int i = n - 1; i > 0; i--) seg[i] = f(seg[2 * i], seg[2 * i + 1]);
    }

    Segment_Tree(int m, const Monoid &x, const F &f, const Monoid &e1) : f(f), e1(e1) {
        n = 1;
        while (n < m) n <<= 1;
        seg.assign(2 * n, e1);
        vector<Monoid> v(m, x);
        copy(begin(v), end(v), begin(seg) + n);
        for (int i = n - 1; i > 0; i--) seg[i] = f(seg[2 * i], seg[2 * i + 1]);
    }

    void change(int i, const Monoid &x, bool update = true) {
        if (update) {
            seg[i + n] = x;
        } else {
            seg[i + n] = f(seg[i + n], x);
        }
        i += n;
        while (i >>= 1) { seg[i] = f(seg[2 * i], seg[2 * i + 1]); }
    }

    Monoid query(int l, int r) const {
        Monoid L = e1, R = e1;
        l += n, r += n;
        while (l < r) {
            if (l & 1) L = f(L, seg[l++]);
            if (r & 1) R = f(seg[--r], R);
            l >>= 1, r >>= 1;
        }
        return f(L, R);
    }

    Monoid operator[](int i) const { return seg[n + i]; }

    template <typename C>
    int find_subtree(int i, const C &check, const Monoid &x, Monoid &M, bool type) const {
        while (i < n) {
            Monoid nxt = type ? f(seg[2 * i + type], M) : f(M, seg[2 * i + type]);
            if (check(nxt, x)) {
                i = 2 * i + type;
            } else {
                M = nxt, i = 2 * i + (type ^ 1);
            }
        }
        return i - n;
    }

    template <typename C>
    int find_first(int l, const C &check, const Monoid &x) const { // check((区間[l,r]での演算結果), x)を満たす最小のr
        Monoid L = e1;
        int a = l + n, b = n + n;
        while (a < b) {
            if (a & 1) {
                Monoid nxt = f(L, seg[a]);
                if (check(nxt, x)) return find_subtree(a, check, x, L, false);
                L = nxt, a++;
            }
            a >>= 1, b >>= 1;
        }
        return n;
    }

    template <typename C>
    int find_last(int r, const C &check, const Monoid &x) const { // check((区間[l,r)での演算結果), x)を満たす最大のl
        Monoid R = e1;
        int a = n, b = r + n;
        while (a < b) {
            if (b & 1 || a == 1) {
                Monoid nxt = f(seg[--b], R);
                if (check(nxt, x)) return find_subtree(b, check, x, R, true);
                R = nxt;
            }
            a >>= 1, b >>= 1;
        }
        return -1;
    }
};

template <typename T>
vector<int> manachar(const T &s) {
    int n = s.size();
    vector<int> ret(n);
    int i = 0, j = 0;
    while (i < n) {
        while (i - j >= 0 && i + j < n && s[i - j] == s[i + j]) j++;
        ret[i] = j;
        int k = 1;
        while (i - k >= 0 && k + ret[i - k] < j) ret[i + k] = ret[i - k], k++;
        i += k, j -= k;
    }
    return ret;
}

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

    string X = T;
    reverse(all(X));

    if (M & 1 || T != X) {
        cout << "-1\n";
        return 0;
    }
    M /= 2;

    string U;
    U += T[0];
    rep2(i, 1, M - 1) {
        U += '_';
        U += T[i];
    }

    vector<int> c = manachar(U);
    // print(c);

    auto f = [](int a, int b) { return min(a, b); };
    Segment_Tree<int> seg(M, inf, f, inf);
    seg.change(M - 1, 1);

    rep3(i, M - 2, 0) {
        int x = seg.query(i + 1, i + c[2 * i + 1] / 2 + 1);
        if (x != inf) seg.change(i, x + 1);
    }
    // rep(i, M) cout << seg[i] << ' ';
    // cout << '\n';

    int ans = inf;
    rep(i, M) {
        if (i == N || S[i] != T[i]) break;
        chmin(ans, seg[i]);
    }
    rep(i, M) {
        if (i == N || S[N - 1 - i] != T[i]) break;
        chmin(ans, seg[i]);
    }

    cout << (ans == inf ? -1 : ans) << '\n';
}