#include <bits/stdc++.h>
using namespace std;
//using namespace atcoder;
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define FOR(i, begin, end) for(int i=(begin);i<(end);i++)
#define REP(i, n) FOR(i,0,n)
#define IFOR(i, begin, end) for(int i=(end)-1;i>=(begin);i--)
#define IREP(i, n) IFOR(i,0,n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(),v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x) distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x) distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define chmax(a, b) a = max(a, b)
#define chmin(a, b) a = min(a, b)
#define bit(n) (1LL<<(n))
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v) { cout << #v << "=" << endl; REP(i_debug, (int)v.size()){ cout << v[i_debug] << ","; } cout << endl; }
#define mdebug(m) { cout << #m << "=" << endl; REP(i_debug, (int)m.size()){ REP(j_debug, (int)m[i_debug].size()){ cout << m[i_debug][j_debug] << ","; } cout << endl;} }
#define pb push_back
#define fi first
#define se second
#define int long long
#define INF 1000000000000000000
template<typename T> istream &operator>>(istream &is, vector<T> &v){ for (auto &x : v) is >> x; return is; }
template<typename T> ostream &operator<<(ostream &os, vector<T> &v){ for(int i = 0; i < (int)v.size(); i++) { cout << v[i]; if(i != (int)v.size() - 1) cout << endl; }; return os; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, pair<T1, T2> p){ cout << '(' << p.first << ',' << p.second << ')'; return os; }
template<typename T> void Out(T x) { cout << x << endl; }
template<typename T1, typename T2> void chOut(bool f, T1 y, T2 n) { if(f) Out(y); else Out(n); }

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using v_bool = vector<bool>;
using v_Pii = vector<Pii>;

//int dx[4] = {1,0,-1,0};
//int dy[4] = {0,1,0,-1};
//char d[4] = {'D','R','U','L'};

const int mod = 1000000007;
//const int mod = 998244353;

vector<int> Manacher(string s){
    int n = s.size();
    vector<int> R(n);
    int i = 0, j = 0;
    while(i < n){
        while(i - j >= 0 && i + j < n && s[i - j] == s[i + j]) j++;
        R[i] = j;
        int k = 1;
        while(i - k >= 0 && i + k < n && k + R[i - k] < j){
            R[i + k] = R[i - k];
            k++;
        }
        i += k; j -= k;
    }
    return R;
}

string fill_dammy(string s, char d = '@'){
    int n = s.size();
    string t = "";
    t += d;
    REP(i, n){
        t += s[i];
        t += d;
    }
    return t;
}

struct SegmentTree
{
    using T = int;

    int N;
    vector<T> dat;

    T id = INF;
    T F(T &a, T &b) { return min(a, b); }
    
    SegmentTree(int n){
        N = 1;
        while(n > N) N = N << 1;
        dat = vector<T>(2 * N - 1, id);
    }

    SegmentTree(int n, vector<T> &v){
        N = 1;
        while(n > N) N = N << 1;
        dat = vector<T>(2 * N - 1, id);
        for(int i = 0; i < n; i++) dat[i + N - 1] = v[i];
        for(int i = N - 2; i >= 0; i--) dat[i] = F(dat[i * 2 + 1], dat[i * 2 + 2]); 
    }

    SegmentTree(){}


    void update(int k, T a){
        k += N - 1;
        dat[k] = a;
        while(k > 0){
            k = (k - 1) / 2;
            dat[k] = F(dat[k * 2 + 1], dat[k * 2 + 2]);
        }
    }

    void reset() { fill(dat.begin(), dat.end(), id); }

    T get(int a, int b, int k, int l, int r){
        if(r <= a || b <= l) return id;
        if(a <= l && r <= b) return dat[k];
        else{
            T vl = get(a, b, k * 2 + 1, l, (l + r) / 2);
            T vr = get(a, b, k * 2 + 2, (l + r) / 2, r);
            return F(vl, vr);
        }
    }
    T get(int a, int b) { return get(a, b, 0, 0, N); }

    T val(int k){ return dat[k + N - 1]; }
};

int solve(int N, int M, string S, string T){
    int n = 0;
    while(n < min(N, M) && S[n] == T[n]) n++;
    if(n == 0) return INF;

    auto T2 = fill_dammy(T);
    auto R = Manacher(T2);
    //debug(T2); vdebug(R);

    int ans = INF;
    SegmentTree ST(M + 1);
    ST.update(M, 0);
    IFOR(i, 1, M){
        int x = 2 * i + R[2 * i];
        chmin(x, 2 * M);
        x /= 2;
        int tmp = ST.get(i + 1, x + 1) + 1;
        if(tmp < INF){
            ST.update(i, tmp);
            if(i <= n) chmin(ans, tmp);
        }
    }
    
    return ans;

}

signed main(){

    int N, M; cin >> N >> M;
    string S, T; cin >> S >> T;

    int ans = INF;
    REP(k, 2){
        chmin(ans, solve(N, M, S, T));
        if(k == 0){
            Reverse(S);
            Reverse(T);
        }
    }
    chOut(ans < INF, ans, -1);

    return 0;
}