#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;
	cin>>D>>s>>s1;
	s+=s1;
	queue<int> q;//num of x
	q.push(0);
	int ans=0;
	REP(i,0,14){
		if(s[i]=='x'){
			q.push(i+1);
		}
		if((int)q.size()>D+1){
			ans=max(ans,i-q.front());
			q.pop();
		}
	}
	ans=max(ans,14-q.front());
	p(ans);
	return 0;
}