#include "bits/stdc++.h"
using namespace std;

int main() {
	int D;
	cin >> D;
	string s1, s2;
	cin >> s1 >> s2;
	s1 += s2;
	int ans = 0;

	int cnt2 = 0;
	for (int i = 0; i < s1.size(); i++)
	{
		if (s1[i] == 'o'){
			cnt2++;
			ans = max(cnt2, ans);
		}
		else cnt2 = 0;
	}

	for (int j = -1; j <= 14; j++)
	{
		int tempans = 0;
		int cnt = 0;
		if (j == -1){
			cnt = D;
		}
		for (int i = 0; i < s1.size(); i++)
		{
			if (((j - i) >= 0 && (j - i) < D && j != -1)){
				if (s1[i] == 'o'){
					tempans = -1; break;
				}
				else{
					cnt++;
					tempans = max(tempans, cnt);
				}
			}
			else if (s1[i] == 'o'){
				cnt++;
				tempans = max(tempans, cnt);
			}
			else cnt = 0;
		}
		if (tempans < 0) continue;
		cnt += max(0, D + j - 14);
		tempans = max(cnt, tempans);
		ans = max(ans, tempans);
	}
	cout << ans << endl;
	cin >> ans;
}