#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 = char, typename U = std::string>
std::vector<std::pair<T, int>> run_length_encoding(const U &s) {
  int n = s.size();
  std::vector<std::pair<T, int>> res;
  T now = s[0];
  int cnt = 1;
  for (int i = 1; i < n; ++i) {
    if (s[i] != now) {
      res.emplace_back(now, cnt);
      cnt = 0;
    }
    now = s[i];
    ++cnt;
  }
  res.emplace_back(now, cnt);
  return res;
}

int solve(int n, const string &s, const vector<pair<char, int>> &t) {
  if (s.front() != t.front().first) return INF;
  const int m = t.size();
  auto rle = run_length_encoding(s);
  int pt = 0, ps = 0, ans = 0;
  while (true) {
    while (ps < rle.size() && pt < m && t[pt] == rle[ps]) ++ps, ++pt;
    if (rle.size() > ps + 1) rle.resize(ps + 1);
    if (pt == m) return ps == rle.size() ? ans : ans + 1;
    ++ans;
    if (ps == rle.size()) --ps, --pt;
    if (t[pt].second % 2 == 1) return INF;
    while (rle[ps].second < t[pt].second) {
      rle[ps].second = min(rle[ps].second * 2, t[pt].second);
      if (rle[ps].second < t[pt].second) ++ans;
    }
    rle[ps].second = t[pt].second;
    ++ps;
    ++pt;
    for (int i = ps - 2; i >= 0; --i) {
      rle.emplace_back(rle[i]);
      if (pt == m || rle.back() != t[pt]) break;
      ++ps;
      ++pt;
    }
    // cout << ps << ' ' << pt << " : " << ans << '\n';
    // REP(i, rle.size()) cout << '{' << rle[i].first << ',' << rle[i].second << '}' << " \n"[i + 1 == rle.size()];
  }
  assert(false);
  return INF;
}

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;
  }
  auto rle = run_length_encoding(t);
  int ans = solve(n, s, rle);
  reverse(ALL(s));
  chmin(ans, solve(n, s, rle));
  cout << (ans == INF ? -1 : ans) << '\n';
  return 0;
}