#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define fr(i, a, b) for (int i = (a); i < (b); i++)
using namespace std;
typedef long long ll;
const ll inf = 1e9;
const ll mod = 1e9 + 7;

typedef pair<int, int> P;

class RLE {
public:
    vector<P> encode(vector<int> v) {
        vector<P> res;
        P p(v[0], 1);

        for (int i = 1; i < v.size(); i++) {
            if (p.first == v[i]) {
                p.second++;
            } else {
                res.push_back(p);
                p = make_pair(v[i], 1);
            }
        }
        res.push_back(p);

        return res;
    }
};

int main() {
    string s[2];
    int D;
    cin >> D;
    cin >> s[0] >> s[1];
    string w = s[0] + s[1];

    RLE rle;
    vector<int> u;

    rep (i, 14) {
        u.push_back(w[i] == 'o');
    }

    vector<P> v = rle.encode(u);

    int ans = 0;

    for (int i = 0; i < v.size(); i++) {
        if (v[i].first == 0) {
            int DD = min(v[i].second, D);
            int temp = DD;
            int temp2 = DD;

            if (v[i].second == DD && i + 1 < v.size()) {
                temp += v[i + 1].second;
            }

            if (v[i].second == DD && i - 1 >= 0) {
                temp2 += v[i - 1].second;
            }

            if (i - 1 >= 0) {
                temp += v[i - 1].second;
            }

            if (i + 1 < v.size()) {
                temp2 += v[i + 1].second;
            }

            ans = max(ans, temp);
            ans = max(ans, temp2);
        }
    }

    cout << ans << endl;
}