#define MOD_TYPE 1 #pragma region Macros #include using namespace std; #include using namespace atcoder; #if 0 #include #include using Int = boost::multiprecision::cpp_int; using lld = boost::multiprecision::cpp_dec_float_100; #endif #if 1 #pragma GCC target("avx2") #pragma GCC optimize("O3") #pragma GCC optimize("unroll-loops") #endif using ll = long long int; using ld = long double; using pii = pair; using pll = pair; using pld = pair; template using smaller_queue = priority_queue, greater>; #if MOD_TYPE == 1 constexpr ll MOD = ll(1e9 + 7); #else #if MOD_TYPE == 2 constexpr ll MOD = 998244353; #else constexpr ll MOD = 1000003; #endif #endif using mint = static_modint; constexpr int INF = (int)1e9 + 10; constexpr ll LINF = (ll)4e18; constexpr double PI = acos(-1.0); constexpr double EPS = 1e-11; constexpr int Dx[] = {0, 0, -1, 1, -1, 1, -1, 1, 0}; constexpr int Dy[] = {1, -1, 0, 0, -1, -1, 1, 1, 0}; #define REP(i, m, n) for (ll i = m; i < (ll)(n); ++i) #define rep(i, n) REP(i, 0, n) #define REPI(i, m, n) for (int i = m; i < (int)(n); ++i) #define repi(i, n) REPI(i, 0, n) #define MP make_pair #define MT make_tuple #define YES(n) cout << ((n) ? "YES" : "NO") << "\n" #define Yes(n) cout << ((n) ? "Yes" : "No") << "\n" #define possible(n) cout << ((n) ? "possible" : "impossible") << "\n" #define Possible(n) cout << ((n) ? "Possible" : "Impossible") << "\n" #define Yay(n) cout << ((n) ? "Yay!" : ":(") << "\n" #define all(v) v.begin(), v.end() #define NP(v) next_permutation(all(v)) #define dbg(x) cerr << #x << ":" << x << "\n"; struct io_init { io_init() { cin.tie(0); ios::sync_with_stdio(false); cout << setprecision(30) << setiosflags(ios::fixed); }; } io_init; template inline bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; } template inline bool chmax(T &a, T b) { if (a < b) { a = b; return true; } return false; } inline ll CEIL(ll a, ll b) { return (a + b - 1) / b; } template inline void Fill(A (&array)[N], const T &val) { fill((T *)array, (T *)(array + N), val); } template vector compress(vector &v) { vector val = v; sort(all(val)), val.erase(unique(all(val)), val.end()); for (auto &&vi : v) vi = lower_bound(all(val), vi) - val.begin(); return val; } template constexpr istream &operator>>(istream &is, pair &p) noexcept { is >> p.first >> p.second; return is; } template constexpr ostream &operator<<(ostream &os, pair p) noexcept { os << p.first << " " << p.second; return os; } ostream &operator<<(ostream &os, mint m) { os << m.val(); return os; } random_device seed_gen; mt19937_64 engine(seed_gen()); struct BiCoef { vector fact_, inv_, finv_; BiCoef(int n) noexcept : fact_(n, 1), inv_(n, 1), finv_(n, 1) { fact_.assign(n, 1), inv_.assign(n, 1), finv_.assign(n, 1); for (int i = 2; i < n; i++) { fact_[i] = fact_[i - 1] * i; inv_[i] = -inv_[MOD % i] * (MOD / i); finv_[i] = finv_[i - 1] * inv_[i]; } } mint C(ll n, ll k) const noexcept { if (n < k || n < 0 || k < 0) return 0; return fact_[n] * finv_[k] * finv_[n - k]; } mint P(ll n, ll k) const noexcept { return C(n, k) * fact_[k]; } mint H(ll n, ll k) const noexcept { return C(n + k - 1, k); } mint Ch1(ll n, ll k) const noexcept { if (n < 0 || k < 0) return 0; mint res = 0; for (int i = 0; i < n; i++) res += C(n, i) * mint(n - i).pow(k) * (i & 1 ? -1 : 1); return res; } mint fact(ll n) const noexcept { if (n < 0) return 0; return fact_[n]; } mint inv(ll n) const noexcept { if (n < 0) return 0; return inv_[n]; } mint finv(ll n) const noexcept { if (n < 0) return 0; return finv_[n]; } }; BiCoef bc(500010); #pragma endregion // 01BFS struct BFS { vector d; vector>> E; BFS(int V) : d(V, INF), E(V) {} void add_edge(int u, int v, bool c, bool directed = true) { E[u].push_back({v, c}); if (!directed) E[v].push_back({u, c}); } void calc(int s) { d[s] = 0; deque que; que.push_front(s); while (!que.empty()) { int u = que.front(); que.pop_front(); for (auto [v, c] : E[u]) { if (d[v] <= d[u] + c) continue; d[v] = d[u] + c; if (!c) que.push_front(v); else que.push_back(v); } } } }; // 偶回文半径 // res[i] : i と i+1 の間を中心とする回文の長さの Max/2 vector manachar_even(string &s_) { string s; s.push_back(s_[0]); REP(i, 1, s_.length()) { s.push_back('$'); s.push_back(s_[i]); } vector mch(s.length()), res; int i = 0, j = 0; while (i < s.size()) { while (i - j >= 0 && i + j < s.size() && s[i - j] == s[i + j]) ++j; mch[i] = j; int k = 1; while (i - k >= 0 && k + mch[i - k] < j) mch[i + k] = mch[i - k], ++k; i += k, j -= k; } for (int i = 1; i < mch.size(); i += 2) res.push_back(mch[i] / 2); return res; } void solve() { int n, m; string s, t; cin >> n >> m >> s >> t; string u = t; reverse(all(u)); if (t.length() % 2 == 1 or t != u) { cout << -1 << "\n"; return; } t = t.substr(0, t.length() / 2); m /= 2; BFS bfs(m + 1); const int S = m; repi(i, min(m, n)) { if (s[i] != t[i]) break; bfs.add_edge(S, i, 1); } repi(i, min(m, n)) { if (s[n - i - 1] != t[i]) break; bfs.add_edge(S, i, 1); } auto r = manachar_even(t); rep(i, m - 1) { bfs.add_edge(i + 1, i, 0); if (r[i] > 0) bfs.add_edge(i, i + r[i], 1); } bfs.calc(S); int ans = bfs.d[m - 1]; cout << (ans == INF ? -1 : ans) << "\n"; } int main() { solve(); }