#include <bits/stdc++.h>

#define rep(i, n) for (int (i) = 0; (i) < (int)(n); (i)++)

const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<int, int> pii;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int D;
    cin >> D;
    string s, t;
    cin >> s >> t;
    s += t;
    int ans = 0;
    for (int j = 0; j + D <= 14; j++) {
        string ss = s;
        for (int i = j; i < j+D; i++) {
            if (s[i] == 'o') break;
            ss[i] = 'o';
        }
        int cur = 0;
        while (cur < 14) {
            int i;
            for (i = cur; i < 14; i++) {
                if (ss[i] != 'o') break;
            }
            ans = max(ans, i-cur);
            cur = i+1;
        }
    }
    int i;
    for (i = 0; i < 14; i++) {
        if (s[i] != 'o') break;
    }
    ans = max(ans, i+D);
    for (i = 13; i >= 0; i--) {
        if (s[i] != 'o') break;
    }
    ans = max(ans, 13+D-i);
    cout << ans << endl;
    return 0;
}