#include <iostream>
#include <algorithm>
using namespace std;

string conv(string str){
  while(str[0] == 'x')
    str = str.substr(1, str.size() - 1);
  while(str[str.size() - 1] == 'x')
    str = str.substr(0, str.size() - 1);

  return str;
}

int solve(string str, int d){
  str = conv(str);
  if(str.size() == 0)
    return d;
  vector<int> vec;
  int tmp = 1;
  for(int i = 1; i < str.size(); i++){
    if(str[i] == str[i - 1])
      tmp++;
    else {
      vec.push_back(tmp);
      tmp = 1;
    }
  }
  vec.push_back(tmp);

  if(vec.size() == 1)
    return vec[0] + d;

  int result;
  result = vec[0] + d;
  result = max(result, vec[vec.size() - 1] + d);
  for(int i = 1; i < vec.size(); i += 2){
    if(vec[i] <= d)
      result = max(result, vec[i - 1] + vec[i] + vec[i + 1]);
    else {
      result = max(result, vec[i - 1] + d);
      result = max(result, vec[i + 1] + d);
    }
  }

  return result;
}

int main(){
  int d;
  cin >> d;
  string a, b;
  cin >> a >> b;
  string str = a + b;
  cout << solve(str, d) << endl;
}