#include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()

template<typename A, typename B> inline bool chmax(A &a, B b) { if (a<b) { a=b; return 1; } return 0; }
template<typename A, typename B> inline bool chmin(A &a, B b) { if (a>b) { a=b; return 1; } return 0; }

typedef unsigned long long ull;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const ll INF = 1ll<<29;
const ll MOD = 1000000007;
const double EPS  = 1e-10;

int main() {
	int d;
	cin >> d;
	
	string s, tmp;
	cin >> s >> tmp;
	s += tmp;
	
	s = "xxxxxxxxxxxxxxx" + s;
	s += "xxxxxxxxxxxxxxx";
	
	int dp[2][100] = {};
	
	REP(i, s.size()) {
		if (s[i] == 'o') {
			chmax(dp[1][i + 1], dp[1][i] + 1);
			chmax(dp[0][i + 1], dp[0][i] + 1);
		}
		else {
			REP(j, d + 1) {
				if (i + j >= s.size()) break;
				
				chmax(dp[1][i + j], dp[0][i] + j);
				if (s[i + j] == 'o') break;
			}
		}
	}
	
	int ans = 0;
	REP(i, 100) chmax(ans, dp[1][i]);
	cout << ans << endl;
	
	return 0;
}