#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;

struct Manacher {
  template <typename T>
  explicit Manacher(const T& s) {
    T str;
    int n = s.size();
    str.reserve(n * 2 + 1);
    for (int i = 0; i < n; ++i) {
      str.push_back(s[i]);
      str.push_back('$');
    }
    str.pop_back();
    n = str.size();
    radius.resize(n);
    for (int i = 0, j = 1; i < n;) {
      while (i - j >= 0 && i + j < n && str[i - j] == str[i + j]) ++j;
      radius[i] = j;
      int k = 1;
      for (; i - k >= 0 && i + k < n && k + radius[i - k] < j; ++k) {
        radius[i + k] = radius[i - k];
      }
      i += k;
      j -= k;
    }
  }

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

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

  bool is_palindrome(const int left, const int right) const {
    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(const string &s, const string &t) {
  const int n = s.length(), m = t.length();
  const Manacher manacher(t);
  vector<int> dp(m + 1, INF);
  deque<int> que;
  for (int i = 0; i < n && i < m && s[i] == t[i]; ++i) {
    dp[i + 1] = 1;
    que.emplace_back(i + 1);
  }
  while (!que.empty()) {
    const int i = que.front(); que.pop_front();
    if (i > 0) {
      if (chmin(dp[i - 1], dp[i])) que.emplace_front(i - 1);
      const int r = manacher.even(i - 1);
      if (chmin(dp[i + r], dp[i] + 1)) que.emplace_back(i + r);
    }
  }
  return dp[m];
}

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;
  }
  t.resize(m / 2);
  int ans = solve(s, t);
  ranges::reverse(s);
  chmin(ans, solve(s, t));
  cout << (ans == INF ? -1 : ans) << '\n';
  return 0;
}