#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int DY[]{1, 0, -1, 0}, DX[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1}, DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

template <typename T>
struct LazySegmentTree {
  using Monoid = typename T::Monoid;
  using OperatorMonoid = typename T::OperatorMonoid;

  LazySegmentTree(int n) : LazySegmentTree(std::vector<Monoid>(n, T::m_id())) {}

  LazySegmentTree(const std::vector<Monoid> &a) : n(a.size()) {
    while ((1 << height) < n) ++height;
    p2 = 1 << height;
    lazy.assign(p2, T::o_id());
    dat.assign(p2 << 1, T::m_id());
    for (int i = 0; i < n; ++i) dat[i + p2] = a[i];
    for (int i = p2 - 1; i > 0; --i) dat[i] = T::m_merge(dat[i << 1], dat[(i << 1) + 1]);
  }

  void set(int idx, const Monoid val) {
    idx += p2;
    for (int i = height; i > 0; --i) propagate(idx >> i);
    dat[idx] = val;
    for (int i = 1; i <= height; ++i) {
      int current_idx = idx >> i;
      dat[current_idx] = T::m_merge(dat[current_idx << 1], dat[(current_idx << 1) + 1]);
    }
  }

  void apply(int idx, const OperatorMonoid val) {
    idx += p2;
    for (int i = height; i > 0; --i) propagate(idx >> i);
    dat[idx] = T::apply(dat[idx], val);
    for (int i = 1; i <= height; ++i) {
      int current_idx = idx >> i;
      dat[current_idx] = T::m_merge(dat[current_idx << 1], dat[(current_idx << 1) + 1]);
    }
  }

  void apply(int left, int right, const OperatorMonoid val) {
    if (right <= left) return;
    left += p2;
    right += p2;
    int left_ctz = __builtin_ctz(left);
    for (int i = height; i > left_ctz; --i) propagate(left >> i);
    int right_ctz = __builtin_ctz(right);
    for (int i = height; i > right_ctz; --i) propagate(right >> i);
    for (int l = left, r = right; l < r; l >>= 1, r >>= 1) {
      if (l & 1) sub_apply(l++, val);
      if (r & 1) sub_apply(--r, val);
    }
    for (int i = left >> (left_ctz + 1); i > 0; i >>= 1) dat[i] = T::m_merge(dat[i << 1], dat[(i << 1) + 1]);
    for (int i = right >> (right_ctz + 1); i > 0; i >>= 1) dat[i] = T::m_merge(dat[i << 1], dat[(i << 1) + 1]);
  }

  Monoid get(int left, int right) {
    if (right <= left) return T::m_id();
    left += p2;
    right += p2;
    int left_ctz = __builtin_ctz(left);
    for (int i = height; i > left_ctz; --i) propagate(left >> i);
    int right_ctz = __builtin_ctz(right);
    for (int i = height; i > right_ctz; --i) propagate(right >> i);
    Monoid l_res = T::m_id(), r_res = T::m_id();
    for (; left < right; left >>= 1, right >>= 1) {
      if (left & 1) l_res = T::m_merge(l_res, dat[left++]);
      if (right & 1) r_res = T::m_merge(dat[--right], r_res);
    }
    return T::m_merge(l_res, r_res);
  }

  Monoid operator[](const int idx) {
    int node = idx + p2;
    for (int i = height; i > 0; --i) propagate(node >> i);
    return dat[node];
  }

  template <typename G>
  int find_right(int left, G g) {
    if (left >= n) return n;
    left += p2;
    for (int i = height; i > 0; --i) propagate(left >> i);
    Monoid val = T::m_id();
    do {
      while (!(left & 1)) left >>= 1;
      Monoid nx = T::m_merge(val, dat[left]);
      if (!g(nx)) {
        while (left < p2) {
          propagate(left);
          left <<= 1;
          nx = T::m_merge(val, dat[left]);
          if (g(nx)) {
            val = nx;
            ++left;
          }
        }
        return left - p2;
      }
      val = nx;
      ++left;
    } while (__builtin_popcount(left) > 1);
    return n;
  }

  template <typename G>
  int find_left(int right, G g) {
    if (right <= 0) return -1;
    right += p2;
    for (int i = height; i > 0; --i) propagate((right - 1) >> i);
    Monoid val = T::m_id();
    do {
      --right;
      while (right > 1 && (right & 1)) right >>= 1;
      Monoid nx = T::m_merge(dat[right], val);
      if (!g(nx)) {
        while (right < p2) {
          propagate(right);
          right = (right << 1) + 1;
          nx = T::m_merge(dat[right], val);
          if (g(nx)) {
            val = nx;
            --right;
          }
        }
        return right - p2;
      }
      val = nx;
    } while (__builtin_popcount(right) > 1);
    return -1;
  }

private:
  int n, p2, height = 0;
  std::vector<Monoid> dat;
  std::vector<OperatorMonoid> lazy;

  void sub_apply(int idx, const OperatorMonoid &val) {
    dat[idx] = T::apply(dat[idx], val);
    if (idx < p2) lazy[idx] = T::o_merge(lazy[idx], val);
  }

  void propagate(int idx) {
    // assert(1 <= idx && idx < p2);
    sub_apply(idx << 1, lazy[idx]);
    sub_apply((idx << 1) + 1, lazy[idx]);
    lazy[idx] = T::o_id();
  }
};

namespace monoid {
template <typename T>
struct RangeMinimumAndUpdateQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() { return std::numeric_limits<Monoid>::max(); }
  static constexpr OperatorMonoid o_id() { return std::numeric_limits<OperatorMonoid>::max(); }
  static Monoid m_merge(const Monoid &a, const Monoid &b) { return std::min(a, b); }
  static OperatorMonoid o_merge(const OperatorMonoid &a, const OperatorMonoid &b) { return b == o_id() ? a : b; }
  static Monoid apply(const Monoid &a, const OperatorMonoid &b) { return b == o_id()? a : b; }
};

template <typename T>
struct RangeMaximumAndUpdateQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() { return std::numeric_limits<Monoid>::lowest(); }
  static constexpr OperatorMonoid o_id() { return std::numeric_limits<OperatorMonoid>::lowest(); }
  static Monoid m_merge(const Monoid &a, const Monoid &b) { return std::max(a, b); }
  static OperatorMonoid o_merge(const OperatorMonoid &a, const OperatorMonoid &b) { return b == o_id() ? a : b; }
  static Monoid apply(const Monoid &a, const OperatorMonoid &b) { return b == o_id()? a : b; }
};

template <typename T, T Inf>
struct RangeMinimumAndAddQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() { return Inf; }
  static constexpr OperatorMonoid o_id() { return 0; }
  static Monoid m_merge(const Monoid &a, const Monoid &b) { return std::min(a, b); }
  static OperatorMonoid o_merge(const OperatorMonoid &a, const OperatorMonoid &b) { return a + b; }
  static Monoid apply(const Monoid &a, const OperatorMonoid &b) { return a + b; }
};

template <typename T, T Inf>
struct RangeMaximumAndAddQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() { return -Inf; }
  static constexpr OperatorMonoid o_id() { return 0; }
  static Monoid m_merge(const Monoid &a, const Monoid &b) { return std::max(a, b); }
  static OperatorMonoid o_merge(const OperatorMonoid &a, const OperatorMonoid &b) { return a + b; }
  static Monoid apply(const Monoid &a, const OperatorMonoid &b) { return a + b; }
};

template <typename T>
struct RangeSumAndUpdateQuery {
  using Monoid = struct {
    T sum;
    int len;
  };
  using OperatorMonoid = T;
  static std::vector<Monoid> init(int n) { return std::vector<Monoid>(n, Monoid{0, 1}); }
  static constexpr Monoid m_id() { return {0, 0}; }
  static constexpr OperatorMonoid o_id() { return std::numeric_limits<OperatorMonoid>::max(); }
  static Monoid m_merge(const Monoid &a, const Monoid &b) { return Monoid{a.sum + b.sum, a.len + b.len}; }
  static OperatorMonoid o_merge(const OperatorMonoid &a, const OperatorMonoid &b) { return b == o_id() ? a : b; }
  static Monoid apply(const Monoid &a, const OperatorMonoid &b) { return Monoid{b == o_id() ? a.sum : b * a.len, a.len}; }
};

template <typename T>
struct RangeSumAndAddQuery {
  using Monoid = struct {
    T sum;
    int len;
  };
  using OperatorMonoid = T;
  static std::vector<Monoid> init(int n) { return std::vector<Monoid>(n, Monoid{0, 1}); }
  static constexpr Monoid m_id() { return {0, 0}; }
  static constexpr OperatorMonoid o_id() { return 0; }
  static Monoid m_merge(const Monoid &a, const Monoid &b) { return Monoid{a.sum + b.sum, a.len + b.len}; }
  static OperatorMonoid o_merge(const OperatorMonoid &a, const OperatorMonoid &b) { return a + b; }
  static Monoid apply(const Monoid &a, const OperatorMonoid &b) { return Monoid{a.sum + b * a.len, a.len}; }
};
}  // monoid

struct Manacher {
  template <typename T>
  Manacher(const T &s) {
    T str;
    int n = s.size();
    for (int i = 0; i < n; ++i) {
      str.push_back(s[i]);
      if (i + 1 < n) str.push_back('$');
    }
    n = str.size();
    radius.resize(n);
    int j = 1;
    for (int i = 0; i < n;) {
      while (i - j >= 0 && i + j < n && str[i - j] == str[i + j]) ++j;
      radius[i] = j;
      int k = 1;
      while (i - k >= 0 && i + k < n && k + radius[i - k] < j) {
        radius[i + k] = radius[i - k];
        ++k;
      }
      i += k;
      j -= k;
    }
  }

  int odd(int idx) const { return (radius[idx * 2] + 1) / 2; }

  int even(int idx) const { return radius[idx * 2 + 1] / 2; }

  bool is_palindrome(int left, int right) const {
    int mid = (left + right - 1) / 2;
    return (((right - left) & 1) ? odd(mid) * 2 - 1 : even(mid) * 2) >= right - left;
  }

private:
  std::vector<int> radius;
};

int solve(int n, const string &s, int m, const string &t) {
  int same = 0;
  for (; same < n && same < m; ++same) {
    if (s[same] != t[same]) break;
  }
  if (same == 0) return INF;
  Manacher manacher(t);
  LazySegmentTree<monoid::RangeMinimumAndAddQuery<int, INF>> seg(m - 1);
  REP(i, m - 1) seg.set(i, m - 1 - i - manacher.even(i));
  int ans = 0;
  for (; m > same; ++ans) {
    int pos = seg.find_right(0, [](int x) -> bool { return x > 0; });
    if (pos >= m - 1) return INF;
    for (; m - 1 > pos; --m) seg.apply(0, m - 1, -1);
  }
  return ans;
}

int main() {
  int n, m; string s, t; cin >> n >> m >> s >> t;
  if (m % 2 == 1 || !equal(ALL(t), t.rbegin())) {
    cout << "-1\n";
    return 0;
  }
  int ans = solve(n, s, m, t);
  reverse(ALL(s));
  chmin(ans, solve(n, s, m, t));
  cout << (ans == INF ? -1 : ans) << '\n';
  return 0;
}