#include <cstdio>
#include <cmath>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <string>
using namespace std;
int d;
string s,ss;

int ans = 0;

void ch(int n){
  int c = n;
  bool f = false;
  for(int i = 0; i < (int)s.size(); i++){
    if(s[i] == 'x'){
      f = true;
      ans = max(ans,c);
      c = 0;
    }else{
      c++;
    }
  }
  ans = max(ans,c+(f?n:0));
}

void solve(int k,int n){
  if(k >= (int)s.size()) ch(n);
  else if(s[k] == 'o') solve(k+1,n);
  else{
    solve(k+1,n);
    if(n){
      s[k] = 'o';
      solve(k+1,n-1);
      s[k] = 'x';
    }
  }
}

int main(void){
  cin >> d;
  cin >> s >> ss;
  s += ss;
  solve(0,d);
  cout << ans << endl;
}