#include <bits/stdc++.h>

using namespace std;

namespace {

    typedef double real;
    typedef long long ll;

    template<class T> ostream& operator<<(ostream& os, const vector<T>& vs) {
        if (vs.empty()) return os << "[]";
        os << "[" << vs[0];
        for (int i = 1; i < vs.size(); i++) os << " " << vs[i];
        return os << "]";
    }
    template<class T> istream& operator>>(istream& is, vector<T>& vs) {
        for (auto it = vs.begin(); it != vs.end(); it++) is >> *it;
        return is;
    }

    int D;
    string s;
    void input() {
        cin >> D;
        cin >> s;
        string t; cin >> t;
        s += t;
        s = string(20, 'x') + s + string(20, 'x');
    }

    void solve() {
        int ans = 0;
        int N = s.size();
        for (int i = 0; i < N; i++) {
            for (int j = i + 1; j <= N; j++) {
                bool flag = true;
                int z = -1;
                for (int k = i; k < j; k++) {
                    if (s[k] == 'x') {
                        if (z < 0) {
                            z = k;
                        } else if (z + D < k) {
                            flag = false;
                        }
                    }
                }
                if (flag) {
                    ans = max(ans, j - i);
                }
            }
        }
        cout << ans << endl;
    }
}

int main() {
    input(); solve();
    return 0;
}