#include <iostream>
#include <vector>
#include <tuple>
#include <algorithm>
using namespace std;
int main(){
  int d;
  cin>>d;
  string s,s2,s3,s4;
  cin>>s;
  cin>>s2;
  s+=s2;
  int f=s.find_first_of('o');
  int l=s.find_last_of('o');
  s3=s;
  s4=s;
  for(int i=0; i<f; ++i){
    s3.erase(s3.begin());
  }
  for(int i=0; i<d; ++i){
    s3.insert(s3.begin(),'o');
  }
  s4.erase(l+1);
  for(int i=0; i<d; ++i){
    s4.append(1,'o');
  }
  int m1=0;
  int a=0;
  for(int i; i<=s3.size(); ++i){
    if(s3[i]=='o'){
      a+=1;
    } else {
      if(a>m1) m1=a;
      a=0;
    }
  }
  int m2=0;
  int b=0;
  for(int i; i<=s4.size(); ++i){
    if(s4[i]=='o'){
      b+=1;
    } else {
      if(b>m2) m2=b;
      b=0;
    }
  }
  int c=0,d2,e;
  vector<tuple<int,int,int>> v;
  for(int i=0; i<s.size(); ++i){
    if(c==0 && s[i]=='x'){
      d2=i;
      c=1;
    } else if(c==1 && s[i]=='o'){
      e=i;
      c=0;
      if(e-d2<=d){
        tuple<int,int,int> t=make_tuple(e-d2,d2,e-1);
        v.push_back(t);
      } else {
        tuple<int,int,int> t=make_tuple(d,d2,d2+d-1);
        v.push_back(t);
        t=make_tuple(d,e-1-d+1,e-1);
        v.push_back(t);
      }
    }
  }
  if(c==1){
    e=s.size();
    c=0;
    if(e-d2<=d){
      tuple<int,int,int> t=make_tuple(e-d2,d2,e-1);
      v.push_back(t);
    } else {
      tuple<int,int,int> t=make_tuple(d,d2,d2+d-1);
      v.push_back(t);
      t=make_tuple(d,e-1-d+1,e-1);
      v.push_back(t);
    }
  }
  int m3=0;
  for(int i=0; i<v.size(); ++i){
    string s4=s;
    int f1=get<1>(v[i]);
    int f2=get<2>(v[i]);
    for(int i2=f1; i2<=f2; ++i2){
      s4[i2]='o';
    }
    int g=0;
    for(int i2=0; i2<s4.size(); ++i2){
      if(s4[i2]=='o'){
        g+=1;
      } else {
        if(g>m3) m3=g;
        g=0;
      }
    }
    if(g!=0){
      if(g>m3) m3=g;
      g=0;
    }
  }
  int m=max(m1,max(m2,m3));
  cout<<m<<endl;
  return 0;
}