#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 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);
    int INF = 1<<30;
    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>>ev(M,vector<int>());
    multiset<int>vs;

    for(int i=0;i<M;i++){
        for(int x : ev[i]){
            if(x > 0){
                vs.insert(x);
            }
            else{
                vs.erase(vs.find(-x));
            }
        }
        if(vs.size()>0)dp[i] = min(dp[i],*vs.begin());

        if(R[i]>0){
            if(i+1<M)ev[i+1].push_back(dp[i]+1);
            if(i+R[i]+1<M)ev[i+R[i]+1].push_back(-(dp[i]+1));
        }
    }
    
    int ans = dp[M-1];
    if(ans == INF)ans = -1;
    cout << ans << endl;
    

    return 0;
}