#include <map>
#include <set>
#include <list>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <string>
#include <vector>
#include <complex>
#include <cstdlib>
#include <cstring>
#include <numeric>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <functional>
 
#define mp       make_pair
#define pb       push_back
#define all(x)   (x).begin(),(x).end()
#define rep(i,n) for(int i=0;i<(n);i++)
 
using namespace std;
 
typedef    long long          ll;
typedef    unsigned long long ull;
typedef    vector<bool>       vb;
typedef    vector<int>        vi;
typedef    vector<vb>         vvb;
typedef    vector<vi>         vvi;
typedef    pair<int,int>      pii;
 
const int INF=1<<29;
const double EPS=1e-9;
 
const int dx[]={1,0,-1,0},dy[]={0,-1,0,1};

int kyuujitu(string s) {
    int count = 0;
    int ma = 0;
    for (int i = 0; i < s.size(); ++i) {
        if (s[i] == 'o') {
            count++;
        } else {
            ma = max(ma, count);
            count = 0;
        }
    }
    return max(ma, count);
}

int main(int argc, char const *argv[]) {
    
    int D;
    cin >> D;
    string s1, s2;
    cin >> s1 >> s2;
    s1 += s2;
    s1 = "xxxxxxxxxxxxxx" + s1 + "xxxxxxxxxxxxxx";
    string s3 = s1;
    int ans = 0;
    if (D == 0) {
    	cout << kyuujitu(s1) << endl;
    	return 0;
    }
    for (int i = 0; i < (s1.size() - D + 1); ++i) {
        s1 = s3;
        for (int j = 0; j < D; ++j) {
        	if (s1[i + j] == 'o') break;
            s1[i + j] = 'o';
        }
        ans = max(ans, kyuujitu(s1));
    }
    cout << ans << endl;

    return 0;
}