#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long ll;
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;
    }

    //貪欲に伸ばす
    int itr = -1;
    for(int i=M-1;i>=0;i--){
        if(dp[i] == 1){
            itr = i;
            break;
        }
    }
    if(itr == -1){
        cout << -1 << endl;
        return 0;
    }
    //cout << itr << endl;
    while (true)
    {
        int c = dp[itr];
        int nitr = -1;
        for(int i=0;itr+i+1<M;i++){
            if(S[itr-i] == S[itr+1 + i]){
                dp[itr + 1 + i] = c + 1;
                nitr = itr + 1 + i;
            }
            else{
                break;
            }
        }
        if(nitr==-1){
            cout << -1 << endl;
            return 0;
        }
        itr = nitr;
        //cout << "nitr = " << nitr << endl;
        if(itr == M - 1){
            break;
        }
    }
    cout << dp[M-1] << endl;
    


    return 0;
}