#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long ll;
const int INF = 1<<30;
vector<int> manacher(string s){
    int i = 0, j = 0;
    vector<int>ret(s.size());
    while(i < s.size()){
        while(i-j>=0&&i+j<s.size()&&s[i-j]==s[i+j])j++;
        ret[i] = j;
        int k = 1;
        while(i-k>=0&&k+ret[i-k]<j)ret[i+k]=ret[i-k],++k;
        i+=k;j-=k;
    }
    return ret;
}
int mn(int l,int r){
    return min(l,r);
}
int inf(){
    return INF;
}
int main(){
    int N, M;
    cin >> N >> M;
    string S, T;
    cin >> S;
    cin >> T;

    if(M%2==1){cout << -1 << endl;return 0;}
    for(int i=0;i<M/2;i++){
        if(T[i]!=T[M-1-i]){
            cout << -1 << endl;
            return 0;
        }
    }
    M/=2;
    T = T.substr(0,M);
    vector<int>dp(M,INF);
    for(int i=0;i<N;i++){
        if(i>=M||S[i]!=T[i])break;
        dp[i] = 1;
    }
    for(int i=0;i<N;i++){
        if(i>=M||S[N-1-i]!=T[i])break;
        dp[i] = 1;
    }
    
    string tt = "";
    for(int i=0;i<M;i++){
        tt += T[i];
        tt += '@';
    }
    vector<int>mc = manacher(tt);
    vector<int>R(M);
    for(int i=0;i<M;i++){
        R[i] = mc[2*i+1]/2;
    }

    vector<vector<int>>G(M,vector<int>());
    for(int i=0;i<M-1;i++){
      G[i+1].push_back((i<<1));
      if(R[i]>0){
        G[i].push_back(((i+R[i])<<1)|1);
      }
    }
    deque<int>bfs;
    for(int i=0;i<M;i++){
      if(dp[i]==1){
        bfs.push_front(i);
      }
    }
    while(!bfs.empty()){
      int v = bfs.front();
      bfs.pop_front();

      for(int nxt:G[v]){
        int c = nxt&1;
        int nx = nxt>>1;
        if(dp[nx]>dp[v]+c){
          dp[nx] = dp[v] + c;
          if(c)bfs.push_back(nx);
          else bfs.push_front(nx);
        }
      }
    }

    if(dp[M-1]==INF)cout << -1 << endl;
    else cout << dp[M-1] << endl;
    

    return 0;
}