#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>
#include <string>

using namespace std;

int calc(string &gw, int paidHoliday, vector<int> &normals) {
	int holidays = 0;
	int max_holidays = 0;
	vector<bool> paid(gw.size(), 0);

	int idx = 0;
	while(paidHoliday > 0) {
		if (paidHoliday & 1) {
			paid[normals[idx]] = true;
		}
		idx++;
		paidHoliday >>= 1;
	}

	for (int i = 0; i < gw.size(); i++) {
		if (gw[i] == 'o' || paid[i]) {
			holidays++;
		} else {
			max_holidays = max(max_holidays, holidays);
			holidays = 0;
		}
	}
	max_holidays = max(max_holidays, holidays);
	return max_holidays;
}

int main() {
	string in1, in2;
	int d;

	cin >> d >> in1 >> in2;
	string gw = in1 + in2;

	vector<int> normals;
	for (int i = 0; i < gw.size(); i++) {
		if (gw[i] == 'x') {
			normals.push_back(i);
		}
	}

	if (normals.size() <= d) {
		cout << gw.size() << endl;
		return 0;
	}

	int ans = 0;
	for (int i = (1 << normals.size()) - 1; i >= 0; i--) {
		if (__builtin_popcount(i) > d) {
			continue;
		}

		ans = max(ans, calc(gw, i, normals));
	}


	cout << ans << endl;
	return 0;
}