#include<complex>
#include<vector>
#include<iostream>
#include<stack>
#include<cmath>
#include<set>
#include<map>
#include<queue>
#include<algorithm>

#define FRONT 0x01
#define RIGHT 0x02
#define BACK 0x04
#define LEFT 0x08
#define OVER 0x10

#define sc second
#define fr first

#define REP(i,n) for(int i = 0; i < (int)(n); ++i)

using namespace std;

typedef double elem;
typedef complex<elem> point, vec;
typedef pair<point, point> line, seg, pp;

const double eps = 1.0e-8;
const double pi = acos(-1.0);
const double inf = 1e40;
point base(0,0);

// lZ
inline elem sq(elem a){ return a*a; }
inline elem cq(elem a){ return a*a*a; }

// pxÏ·
elem rad(elem deg){ return (deg/180)*pi; }
elem deg(elem rad){ return (rad*180)/pi; }

// ®¬_Ìô¢AÈÇ
bool eq(elem a, elem b){ return abs(a-b) < eps; }
bool lt(elem a, elem b){ return !eq(a,b) && a < b; }
bool leq(elem a, elem b){ return eq(a,b) || a < b; }
bool gt(elem a, elem b){ return !eq(a,b) && a > b; }
bool geq(elem a, elem b){ return eq(a,b) || a > b; }
bool ltz(elem a){ return lt( a, 0 ); }
bool gtz(elem a){ return gt( a, 0 ); }
bool eqv(vec a, vec b){ return eq( abs(b-a),0); }
bool is_zv(vec v){ return eq(abs(v),0); }
elem emax(elem a, elem b){ return gt(a,b)?a:b; }
elem emin(elem a, elem b){ return lt(a,b)?a:b; }

// _Iy[^
bool far(point a, point b){ return gtz( abs(b-a) ); }
bool near(point a, point b){ return leq( abs(b-a), 0 ); }
elem dot(vec a, vec b){ return (a.real() * b.real() + a.imag() * b.imag() ); }
elem cross(vec a, vec b){ return ( a.real() * b.imag() - a.imag() * b.real() ); }

// a©çbÜÅvñèÌpxAàpA]ñ]
elem varg(vec a, vec b){
  elem ret=arg(a)-arg(b);
  if(lt(ret,0))ret+=2*pi;
  if(gt(ret,2*pi))ret-=2*pi;
  if(eq(ret,2*pi))ret=0;
  return ret;
}
elem arg(vec a, vec b){ return acos( dot(a,b) / ( abs(a) * abs(b) ) ); }
point rot(point p, elem theta){ return p * polar((elem)1.0, theta); }
point rotdeg(point p, elem deg){ return p * polar((elem)1.0, rad(deg)); }

// ñ_Ô£A¼üÆ_ÌÅZ£AüªÆ_ÌÅZ£
elem dist(point a, point b){ return abs(a-b); }
elem dist_l(line l, point x){ return abs(cross(l.sc-l.fr,x-l.fr)) / abs(l.sc-l.fr); }
elem dist_seg(seg s, point x)
{
  if( ltz( dot(s.sc-s.fr,x-s.fr) ) ) return abs(x-s.fr);
  if( ltz( dot(s.fr-s.sc,x-s.sc) ) ) return abs(x-s.sc);
  return dist_l(s,x);
}

// PÊxNgA@üxNgAPÊ@üxNg
vec uvec(vec a){ return a / abs(a); }
vec nmr(vec a){ return a * vec(0,-1); }
vec nml(vec a){ return a * vec(0,1); }
vec unmr(vec a){ return uvec( nmr(a) ); }
vec unml(vec a){ return uvec( nml(a) ); }

// ¼ðA½s»è
bool orth(point a1, point a2, point b1, point b2){ return eq( dot( a2 - a1, b2 - b1 ), 0 ); }
bool orth(vec v1, vec v2){ return eq( dot(v1, v2), 0 ); }
bool prll(point a1, point a2, point b1, point b2){ return eq( cross( a2 - a1, b2 - b1 ), 0 ); }
bool prll(vec v1, vec v2){ return eq( cross(v1, v2), 0 ); }

// CCW oXg¾ªA¸xÉæé
int ccw(point a, point b, point x){
  b -= a;
  x -= a;
  // if( is_zv(b) || is_zv( x ) ) return ERROR;
  if( gtz( cross(b,x) ) ) return LEFT;
  if( ltz( cross(b,x) ) ) return RIGHT;
  if( ltz( dot(b,x) ) ) return BACK;
  if( abs(b) < abs(x) ) return FRONT;
  return OVER;
}

bool intersected_seg(seg a, seg b)
{
  if( ccw(a.fr,a.sc,b.fr)&OVER || ccw(a.fr,a.sc,b.sc)&OVER ) return true;
  return
    ( ccw(a.fr,a.sc,b.fr) | ccw(a.fr,a.sc,b.sc) ) == (LEFT|RIGHT) &&
    ( ccw(b.fr,b.sc,a.fr) | ccw(b.fr,b.sc,a.sc) ) == (LEFT|RIGHT) ;
}
bool intersected_l(line a, line b){
  return !prll(a.fr,a.sc,b.fr,b.sc);
}
point intersection_seg(seg a, seg b)
{
  elem d1 = dist_l(b,a.fr);
  elem d2 = dist_l(b,a.sc);
  return a.fr + ( d1 / (d1 + d2 ) ) * (a.sc-a.fr);
}
point intersection_l(line a, line b)
{
  vec va = a.sc - a.fr;
  vec vb = b.sc - b.fr;
  return a.fr + va * ( cross(vb, b.fr - a.fr) / cross(vb,va) );
}
bool intersection_seg(seg a, seg b, point &ret){
  if( intersected_seg(a,b) ){
    ret = intersection_seg(a,b);
    return true;
  }
  return false;
}
bool intersection_l(line a, line b, point &ret){
  if( intersected_l( a, b ) ){
    ret = intersection_l( a, b );
    return true;
  }
  return false;
}
bool intersection_lseg(line l, seg s, point &ret){
  point tmp;
  if( intersection_l(l,s,tmp) ){
    if(ccw(s.fr,s.sc,tmp)&OVER){
      ret=tmp;
      return true;
    }
  }
  return false;
}

struct Point{
  point p;
  Point():p(0,0){}
  Point(point tp):p(tp){}
  bool operator<(const Point &t)const{
    if(lt(p.real(),t.p.real()))return true;
    if(eq(p.real(),t.p.real())) return lt(p.imag(),t.p.imag());
    return false;
  }
};

struct Node{
  point p;
  set<int> line_id;
  Node(){}
};

struct Seg{
  seg s;
  vector<int> con;
  Seg(){}
  Seg( seg s ): s(s){}
};

typedef vector<Node> Graph;

struct State{
  int now;
  double cost;
  bool operator>(const State &t)const{
    return cost > t.cost;
  }
};

double dijkstra(vector<Seg> &segs, Graph &G, int s){
  double ret =0;
  
  priority_queue< State,vector<State>,greater<State> > pq;
  bool vis[G.size()];
  int pre[G.size()];
  double A[G.size()];

  for(int i = 0; i < G.size(); ++i){
    vis[i]=false;
    A[i]=inf;
    pre[i]=-1;
  }
  
  State init;
  init.now = s;
  init.cost = 0;
  A[s]=0;
  pq.push(init);
  while(!pq.empty()){
    State st_now = pq.top();
    pq.pop();
    int nowid = st_now.now;

    if( vis[nowid] ){
      continue;
    }else{
      vis[nowid]=true;
    }
    
    // cout << vis.size() << endl;
    // cout << "ID : " << st_now.now << ' ' << " POINT : " << G[st_now.now].p << " COST : " << st_now.cost << endl;

    for(set<int>::iterator its = G[nowid].line_id.begin(); its != G[nowid].line_id.end(); ++its){
      int segid = *its;
      for(int i = 0; i < segs[segid].con.size(); ++i){
	int nextid = segs[segid].con[i];
	double nextcost = st_now.cost + abs(G[nextid].p-G[nowid].p);
	Point np = Point( G[nextid].p );

	if( A[nextid] > nextcost ){
	  A[nextid]=nextcost;
	  pre[nextid]=nowid;
	  State nextState;
	  nextState.now = nextid;
	  nextState.cost = nextcost;
	  if( !vis[nextid] )
	    pq.push(nextState);
	}
      }
    }
  }
  
  vector<int> lid;
  for(int i = 0; i < G.size(); ++i){
    ret = max(ret,A[i]);
  }
  for(int i = 0; i < G.size(); ++i){
    if( i == G.size()-1 ) break;
    lid.push_back( i );
  }
  
  for(int i = 0; i < lid.size(); ++i){
    int nowid = lid[i];
    int preid = pre[nowid];
    int b = nowid;
    set<int> aid;
    for(set<int>::iterator its = G[nowid].line_id.begin(); its != G[nowid].line_id.end(); ++its){
      int segid = *its;
      int aleft=-1;
      int aright=-1;
      int afront=-1;
      double dleft=inf;
      double dright=inf;
      double dfront=inf;
      
      for(int i = 0; i < segs[segid].con.size(); ++i){
	int nextid = segs[segid].con[i];
	if( nowid == nextid ) continue;
	if( preid == nextid ) continue;
	
	double D = abs(G[nowid].p-G[nextid].p);
	int cw = ccw(G[preid].p,G[nowid].p,G[nextid].p);

	if( cw & LEFT ){
	  if( dleft > D ){
	    dleft = D;
	    aleft = nextid;
	  }
	}else if( cw & RIGHT ){
	  if( dright > D ){
	    dright = D;
	    aright = nextid;
	  }
	}else if( cw &(FRONT|OVER) ){
	  if( dfront > D ){
	    dfront = D;
	    afront = nextid;
	  }
	}
      }
      
      if( aleft>=0&& pre[aleft]!=nowid)aid.insert(aleft);
      if(aright>=0&&pre[aright]!=nowid)aid.insert(aright);
      if(afront>=0&&pre[afront]!=nowid)aid.insert(afront);
    }
    
    //cout << "B : " << b << endl;
    for(set<int>::iterator its = aid.begin(); its != aid.end(); ++its){
      //cout << *its << endl;
      if( leq( A[*its], A[b] ) ){
	elem dab = abs( G[*its].p - G[b].p );
	if( leq( A[*its], dab + A[b] ) ){
	  double t = (abs(A[b] - A[*its]) + dab ) / 2.;
	  ret = max(ret,t+A[*its]);
	}
      }
    }
    
  }
  return ret;
}

int main()
{
  while(true){
    int N;
    double ans;
    int id = 0;
    vector<Seg> corridor;
    map<Point,int> interid;
    Graph G;
    
    cin >> N;
    if( N == 0 ) break;
    for(int i = 0; i < N; ++i){
      elem x1,y1,x2,y2;
      cin >> x1 >> y1 >> x2 >> y2;
      corridor.push_back( Seg(seg( point(x1,y1), point(x2,y2) )) );
    }
    
    G.resize( N*N+2*N );
    id = 0;
    
    for(int i = 0; i < corridor.size(); ++i){

      int fid;
      int sid;
      Point pf = Point(corridor[i].s.first);
      Point ps = Point(corridor[i].s.second);

      if( interid.find( pf ) == interid.end() ){
	interid[ Point(corridor[i].s.first) ] = id;
	fid=id;
	++id;
      }else fid=interid[Point(corridor[i].s.first)];
      if( interid.find( ps ) == interid.end() ){
	interid[ Point(corridor[i].s.second ) ] = id;
	sid=id;
	++id;
      }else sid=interid[Point(corridor[i].s.second)];
      
      corridor[i].con.push_back( fid );
      corridor[i].con.push_back( sid );
      
      G[fid].p = corridor[i].s.first;
      G[fid].line_id.insert(i);
      G[sid].p = corridor[i].s.second;
      G[sid].line_id.insert(i);
    }
    
    for(int i = 0; i < corridor.size(); ++i){
      for(int j = i+1; j < corridor.size(); ++j){
	point intersection;
	
	if( intersection_seg( corridor[i].s, corridor[j].s,intersection ) ){
	  int tid;
	  
	  if( interid.find( Point(intersection) ) == interid.end() ){
	    interid[ Point(intersection) ] = id;
	    tid=id;
	    ++id;
	  }else{
	    tid = interid[ Point(intersection) ];
	  }	  
	  corridor[i].con.push_back( tid );
	  corridor[j].con.push_back( tid );
	  
	  G[tid].p = intersection;
	  G[tid].line_id.insert(i);
	  G[tid].line_id.insert(j);
	}
      }
    }
    G.resize(id+1);
    
    elem dx,dy;
    cin >> dx >> dy;

    point demon(dx,dy);
    G[id].p=demon;
    
    for(int i = 0; i < corridor.size(); ++i){
      if( ccw( corridor[i].s.first, corridor[i].s.second, demon ) == OVER ){
	G[id].line_id.insert(i);
	corridor[i].con.push_back( id );
      }
    }

    ans = dijkstra( corridor, G, id );
    
    printf("%.8lf\n",ans);
  }
  return 0;
}