#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <chrono>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")


// |as| = n ==> |rs| = 2 n + 1
// [i - rs[i], i + rs[i]] is palindrome for $ as[0] $ as[1] $ ... $ as[n-1] $
// as[i, j): palindrome <=> j - i <= rs[i + j]
template <class String> vector<int> manacher(const String &as) {
  const int n = as.size();
  vector<int> rs(2 * n + 1);
  for (int i = 0, j = 0, k; i <= 2 * n; i += k, j -= k) {
    for (; 0 < i - j && i + j < 2 * n &&
           (!((i + j + 1) & 1) || as[(i - j - 1) >> 1] == as[(i + j + 1) >> 1]);
         ++j) {}
    rs[i] = j;
    for (k = 1; k < j && k + rs[i - k] < j; ++k) rs[i + k] = rs[i - k];
  }
  return rs;
}


int SLen, TLen;
char S[500'010];
char T[500'010];

int main() {
  for (; ~scanf("%d%d", &SLen, &TLen); ) {
    scanf("%s", S);
    scanf("%s", T);
    
    bool isPalT = true;
    isPalT = isPalT && (TLen % 2 == 0);
    for (int l = 0, r = TLen - 1; l < r; ++l, --r) {
      isPalT = isPalT && (T[l] == T[r]);
    }
    int common = 0;
    for (int dir = 0; dir < 2; ++dir) {
      for (int i = 0; ; ++i) {
        if (!(i < SLen && i < TLen && S[i] == T[i])) {
          chmax(common, i);
          break;
        }
      }
      reverse(S, S + SLen);
    }
    
    int ans = -1;
    if (isPalT) {
      const auto rs = manacher(string(T, T + TLen/2));
      // longest even pal suf
      vector<int> fs(TLen/2 + 1, 0);
      for (int i = 0; i <= TLen/2; ++i) chmax(fs[i + rs[i*2] / 2], rs[i*2]);
      for (int i = TLen/2; --i >= 0; ) chmax(fs[i], fs[i + 1] - 2);
//cerr<<"T/2 = "<<string(T,T+TLen/2)<<endl;
//cerr<<"fs = "<<fs<<endl;
      int cost = 1;
      for (int n = TLen/2; ; ) {
//cerr<<"cost = "<<cost<<", n = "<<n<<endl;
        if (n <= common) {
          ans = cost;
          break;
        }
        // fold suf
        if (!fs[n]) break;
        ++cost;
        n -= fs[n] / 2;
      }
    }
    printf("%d\n", ans);
  }
  return 0;
}