#include <bits/stdc++.h>
#define REP(i,n,N) for(int i=(n);i<(int) N;i++)
#define p(s) cout<<(s)<<endl
using namespace std;

int main() {
	int D;
	string s,s1,x;
	x="xxxxxxxxxxxxxx";
	cin>>D>>s>>s1;
	s=x+s+s1+x;
	int ans=0;
	REP (i,0,s.size()-D+1) {
		int cnt = 0;
		int mx = 0;
		s1 = s;
		REP (j,0,D) {
			if (s1[j+i] == 'o') break;
			s1[j+i] = 'o';
		}
		REP (j,0,s1.size()) {
			if (s1[j] == 'o') {
				cnt++;
			} else {
				mx = max(cnt, mx);
				cnt = 0;
			}
		}
		mx = max(cnt, mx);
		ans = max(ans, mx);
	}
	p(ans);
	return 0;
}