#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
#define rep(i,n)for (int i = 0; i < int(n); ++i)
#define rrep(i,n)for (int i = int(n)-1; i >= 0; --i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
template<class T> void chmax(T& a, const T& b) {a = max(a, b);}
template<class T> void chmin(T& a, const T& b) {a = min(a, b);}
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

template <class S, S (*op)(S, S), S (*e)()> struct SparseTable {
    int n;
    std::vector<int> floor_lg;
    std::vector<std::vector<S>> d;
    explicit SparseTable(std::vector<S>&& a): n(a.size()), floor_lg(n + 1) {
        if (n == 0) return;
        int frog = 0;
        for(int i = 1; i <= n; frog++) {
            int j = std::min(n + 1, 2 * i);
            for(; i < j; i++) floor_lg[i] = frog;
        }
        d.resize(frog);
        d[0] = std::move(a);
        for(int p = 0, w = 1; p < frog - 1; p++, w *= 2) {
            int last = n - 2 * w;
            d[p + 1].resize(last + 1);
            for(int i = 0; i <= last; i++) d[p + 1][i] = op(d[p][i], d[p][i + w]);
        }
    }
    explicit SparseTable(const std::vector<S>& a): SparseTable(std::vector<S>(a)) {}

    S query(int l, int r) {
        assert(0 <= l && l <= r && r <= n);
        if (l >= r) return e();
        int w = r - l;
        int p = floor_lg[w];
        return op(d[p][l], d[p][r - (1 << p)]);
    }
};

int op(int x, int y) { return min(x, y); }
int e() { return 1001001001;}

int op2(int x, int y) { return max(x, y);}
int e2() { return -1; }

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n, m;
    cin >> n >> m;
    string s, t;
    cin >> s >> t;
    constexpr int INF = 1001001001;
    int mx = 0;
    rep(_, 2) {
        int l = min(n, m);
        rep(i, l) {
            if (s[i] != t[i]) break;
            chmax(mx, i + 1);
        }
        reverse(all(s));
    }
    t += '$';
    rep(i, m) t += t[m - 1 - i];
    auto sa = suffix_array(t);
    VI sa_inv(2 * m + 1);
    rep(i, 2 * m + 1) sa_inv[sa[i]] = i;
    auto rmq = SparseTable<int, op, e>(lcp_array(t, sa));
    VI d(m + 1);
    for(int i = 1; i < m; i++) {
        int idx1 = sa_inv[i], idx2 = sa_inv[2 * m + 1 - i];
        if (idx1 > idx2) swap(idx1, idx2);
        d[i] = i + rmq.query(idx1, idx2);
    }
    d[m] = m;
    segtree<int, op2, e2> seg(d);
    int r = m;
    for(int t = 1;; t++) {
        int c = (r + 1) / 2;
        int nr = seg.max_right(c, [&](int x) { return x < r; });
        if (nr <= mx) {
            cout << t << endl;
            return 0;
        } else if (nr == r) {
            cout << -1 << endl;
            return 0;
        }
        r = nr;
    }
}