#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);
    int INF = 1<<30;
    vector<int>v(M,INF);
    for(int i=0;i<N;i++){
        if(i>=M||S[i]!=T[i])break;
        v[i] = 1;
    }
    for(int i=0;i<N;i++){
        if(i>=M||S[N-1-i]!=T[i])break;
        v[i] = 1;
    }
    
    lazy_segtree<int,mn,inf,int,mn,mn,inf>dp(v);
    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;
    }

    for(int i=0;i<M;i++){
        if(R[i]==0)continue;
        int m = dp.get(i);
        dp.apply(i+1,min(M,i+1+R[i]),m+1);
    }
    
    int ans = dp.get(M-1);
    if(ans == INF)ans = -1;
    cout << ans << endl;
    

    return 0;
}