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

int main() {
	int D;
	cin >> D;
	string s1, s2;
	cin >> s1 >> s2;
	s1 += s2;
	int ans = 0;
	for (int j = -D; j <= 14; j++)
	{
		int cnt = 0;
		if (j < 0){
			cnt = -j;
		}
		for (int i = 0; i < s1.size(); i++)
		{
			if (s1[i] == 'o' || ((i - j) >= 0 && (i - j) < D)){
				cnt++;
				ans = max(cnt, ans);
			}
			else cnt = 0;
		}
		cnt += max(0, D + j - 14);
		ans = max(cnt, ans);
	}
	cout << ans << endl;
}