#include <bits/stdc++.h>
using namespace std;

int ok( string s, int d ){
  for( int i = 0; i < s.size(); ++i )
    if( s[ i ] == 'x' ){
      int j;
      for( j = i + 1; j < s.size(); ++j )
        if( s[ j ] == 'o' )
          break;
      if( j - i > d )
        return 0;
      for( int k = j + 1; k < s.size(); ++k )
        if( s[ k ] == 'x' )
          return 0;
    }
  return 1;
}

signed main(){
  int D; cin >> D;
  string A, B; cin >> A >> B;
  string w = "xxxxxxxxxxxxxx" + A + B + "xxxxxxxxxxxxxx";
  int ans = 0;
  for( int i = 0; i < w.size(); ++i )
    for( int j = i; j < w.size(); ++j )
      if( ok( w.substr( i, j - i + 1 ), D ) )
        ans = max( ans, j - i + 1 );
  cout << ans << endl;
  return 0;
}