#include<iostream>
using namespace std;
string s,t;int d,m,c[99],f=1,p='x';
main()
{
	cin>>d>>s>>t;s=string(d,'x')+s+t+string(d,'x');
	for(int i=0;i<=s.size()-d;i++){
		int cnt=0;int j=i-1;
		while(j>=0&&s[j]=='o')cnt++,j--;
		j=i+d;
		while(j<=s.size()&&s[j]=='o')cnt++,j++;
		cnt+=d;
		m<cnt&&(m=cnt);
	}
	cout<<m<<endl;
}