#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <numeric>
#define rep(x, to) for (int x = 0; x < (to); x++)
#define REP(x, a, to) for (int x = (a); x < (to); x++)
#define foreach(itr, x) for (typeof((x).begin()) itr = (x).begin(); itr != (x).end(); itr++)

using namespace std;

typedef long long ll;
typedef pair<int, int> PII;
typedef pair<long, long> PLL;

string s, t, x;
int tmp, d;
int ans;


vector<int> vec;

int main() {
	cin >> d;
	cin >> s;
	cin >> t;
	rep(i, 14) x += 'x';
	x += s;
	x += t;
	rep(i, 14) x += 'x';

	x += 't'; //何でもいいo,x以外
	//cout << x << endl;
	rep(i, x.size() - 1) {
		tmp++;
		if (x[i] != x[i+1]) {
			if (x[i] == 'o') {
				vec.push_back(tmp);
			} else {
				vec.push_back(-tmp);
			}
			tmp = 0;
		}
	}
	rep(i, vec.size()) {
		if (vec[i] < 0) {
			//両方にくっつけられる
			if (-vec[i] <= d) {
				int a = i-1>=0 ? abs(vec[i-1]) : 0;
				int b = i+1>=vec.size() ? 0 : abs(vec[i+1]);
				ans = max(ans, a - vec[i] + b);
			} else {
			//左右どっちかにくっつける
				int a = i-1>=0 ? abs(vec[i-1]) : 0;
				int b = i+1>=vec.size() ? 0 : abs(vec[i+1]);
				ans = max(ans, a + d); //左
				ans = max(ans, b + d); //右
			}
		}
		//cout << vec[i] << "-----" << ans << endl;
	}
	cout << ans << endl;
	return 0;
}