#include<bits/stdc++.h>

#define REP(i,s,n) for(int i=s;i<n;i++)
#define rep(i,n) REP(i,0,n)
#define EPS (1e-8)
#define COUNTER_CLOCKWISE 1
#define CLOCKWISE -1 
#define ONLINE_BACK 2
#define ONLINE_FRONT -2
#define ON_SEGMENT 0
#define equals(a,b) (fabs((a)-(b)) < EPS)

using namespace std;

class Point{
public:
  double x,y;

  Point(double x = 0,double y = 0): x(x),y(y){}

  Point operator + (Point p){return Point(x+p.x,y+p.y);}
  Point operator - (Point p){return Point(x-p.x,y-p.y);}
  Point operator * (double a){return Point(a*x,a*y);}
  Point operator / (double a){return Point(x/a,y/a);}
  Point operator * (Point a){ return Point(x * a.x - y * a.y, x * a.y + y * a.x); }

  bool operator < (const Point& p) const{ return !equals(x,p.x)?x<p.x:(!equals(y,p.y)&&y<p.y); }

  bool operator == (const Point& p)const{ return fabs(x-p.x) < EPS && fabs(y-p.y) < EPS; }

};

struct Segment{
  Point p1,p2;
  Segment(Point p1 = Point(),Point p2 = Point()):p1(p1),p2(p2){}
 bool operator == (const Segment& p)const { return ( p.p1 == p1 && p.p2 == p2 ) || ( p.p1 == p2 && p.p2 == p1 ); }
};

typedef Point Vector;
typedef Segment Line;
typedef vector<Point> Polygon;

ostream& operator << (ostream& os,const Point& a){ os << "(" << a.x << "," << a.y << ")"; }

ostream& operator << (ostream& os,const Segment& a){ os << "( " << a.p1 << " , " << a.p2 << " )"; }

double dot(Point a,Point b){ return a.x*b.x + a.y*b.y; }

double cross(Point a,Point b){ return a.x*b.y - a.y*b.x; }

double norm(Point a){ return a.x*a.x+a.y*a.y; }

double abs(Point a){ return sqrt(norm(a)); }

//rad は角度をラジアンで持たせること
Point rotate(Point a,double rad){ return Point(cos(rad)*a.x - sin(rad)*a.y,sin(rad)*a.x + cos(rad)*a.y); }

// 度をラジアンに変換
double toRad(double agl){ return agl*M_PI/180.0; }

// a => prev, b => cur, c=> next
// prev から cur へ行って next へ行く際の角度を求める
double getArg(Point a,Point b,Point c){
  double arg1 = atan2(b.y-a.y,b.x-a.x);
  double arg2 = atan2(c.y-b.y,c.x-b.x);
  double arg = fabs( arg1 - arg2 );
  while( arg > M_PI ) arg -= 2.0 * M_PI;
  return fabs(arg);
}


int ccw(Point p0,Point p1,Point p2){
  Point a = p1-p0;
  Point b = p2-p0;
  if(cross(a,b) > EPS)return COUNTER_CLOCKWISE;
  if(cross(a,b) < -EPS)return CLOCKWISE;
  if(dot(a,b) < -EPS)return ONLINE_BACK;
  if(norm(a) < norm(b))return ONLINE_FRONT;
  return ON_SEGMENT;
}

bool intersectLL(Line l, Line m) {
  return abs(cross(l.p2-l.p1, m.p2-m.p1)) > EPS || // non-parallel
         abs(cross(l.p2-l.p1, m.p1-l.p1)) < EPS;   // same line
}
bool intersectLS(Line l, Line s) {
  return cross(l.p2-l.p1, s.p1-l.p1)*       // s[0] is left of l
         cross(l.p2-l.p1, s.p2-l.p1) < EPS; // s[1] is right of l
}
bool intersectLP(Line l,Point p) {
  return abs(cross(l.p2-p, l.p1-p)) < EPS;
}
bool intersectSS(Line s, Line t) {
  return ccw(s.p1,s.p2,t.p1)*ccw(s.p1,s.p2,t.p2) <= 0 &&
         ccw(t.p1,t.p2,s.p1)*ccw(t.p1,t.p2,s.p2) <= 0;
}
bool intersectSP(Line s, Point p) {
  return abs(s.p1-p)+abs(s.p2-p)-abs(s.p2-s.p1) < EPS; // triangle inequality
}

Point projection(Line l,Point p) {
  double t = dot(p-l.p1, l.p1-l.p2) / norm(l.p1-l.p2);
  return l.p1 + (l.p1-l.p2)*t;
}
Point reflection(Line l,Point p) {
  return p + (projection(l, p) - p) * 2;
}
double distanceLP(Line l, Point p) {
  return abs(p - projection(l, p));
}
double distanceLL(Line l, Line m) {
  return intersectLL(l, m) ? 0 : distanceLP(l, m.p1);
}

double distanceLS(Line l, Line s) {
  if (intersectLS(l, s)) return 0;
  return min(distanceLP(l, s.p1), distanceLP(l, s.p2));
}
double distanceSP(Line s, Point p) {
  Point r = projection(s, p);
  if (intersectSP(s, r)) return abs(r - p);
  return min(abs(s.p1 - p), abs(s.p2 - p));
}

double distanceSS(Line s, Line t) {
  if (intersectSS(s, t)) return 0;
  return min(min(distanceSP(s, t.p1), distanceSP(s, t.p2)),
             min(distanceSP(t, s.p1), distanceSP(t, s.p2)));
}

Point crosspoint(Line l,Line m){
  double A = cross(l.p2-l.p1,m.p2-m.p1);
  double B = cross(l.p2-l.p1,l.p2-m.p1);
  if(abs(A) < EPS && abs(B) < EPS){
    vector<Point> vec;
    vec.push_back(l.p1),vec.push_back(l.p2),vec.push_back(m.p1),vec.push_back(m.p2);
    sort(vec.begin(),vec.end());
    assert(vec[1] == vec[2]); //同じセグメントかもよ
    return vec[1];
    //return m.p1;
  }
  if(abs(A) < EPS)assert(false);
  return m.p1 + (m.p2-m.p1)*(B/A);
}

struct Edge {
  int from,to;
  double cost;
  Edge(int from=0,int to=0,double cost=0):from(from),to(to),cost(cost){}
  bool operator < (const Edge& a)const { return !equals(cost,a.cost) && cost < a.cost; }
};

Point sp;

vector<vector<Edge> > segmentArrangement(vector<Segment> &vs,vector<Point> &ps) {
  ps.push_back(sp);

  rep(i,vs.size()) ps.push_back( (vs[i].p1+vs[i].p2) / 2.0 );

  rep(i,vs.size()) ps.push_back(vs[i].p1), ps.push_back(vs[i].p2);
  rep(i,vs.size()) REP(j,i+1,vs.size()) if(intersectSS(vs[i],vs[j])) ps.push_back(Point(crosspoint(vs[i],vs[j])));

  sort(ps.begin(),ps.end());  
  ps.erase(unique(ps.begin(),ps.end()),ps.end());

  vector<vector<Edge> > ret(ps.size());

  for(int i=0;i<vs.size();i++){
    vector<pair<double,int> > list;
    rep(j,ps.size()) if(intersectSP(vs[i],ps[j]))list.push_back(pair<double,int>(norm(vs[i].p1-ps[j]),j));
    sort(list.begin(),list.end());
    for(int j=0;j+1<list.size();++j) {
      int from = list[j].second, to = list[j+1].second;
      double cost = abs(ps[from]-ps[to]);
      ret[from].push_back(Edge(from,to,cost));
      ret[to].push_back(Edge(to,from,cost));
    }
  }  
  return ret;
}

int E;
vector<Segment> segs;

bool  LT(double a,double b) { return !equals(a,b) && a < b; }
bool LTE(double a,double b) { return  equals(a,b) || a < b; }

// --作成中-- 

Point vir;
bool comp_dist(const Point &a,const Point &b){ return LT(abs(vir-a),abs(vir-b)); }

const int MAX_N = 2000;
int uf[MAX_N];
void init(int N=MAX_N) { rep(i,N) uf[i] = i; }
int find(int x) { 
  if( x == uf[x] ) return x;
  return uf[x] = find(uf[x]);
}
void unit(int x,int y){
  x = find(x), y = find(y);
  if( x != y ) uf[x] = y;  
}

void segmentMerge(vector<Segment> &vec,vector<Segment> &ret){
  init(vec.size());
  rep(i,vec.size()) {
    rep(j,vec.size()) {
      if( equals(cross(vec[i].p1-vec[i].p2,vec[j].p1-vec[j].p2),0.0) && intersectSS(vec[i],vec[j]) ) unit(i,j);
    }
  }

  ret.clear();
  vector<int> color;
  rep(i,vec.size()) color.push_back(find(i));
  sort(color.begin(),color.end());
  color.erase(unique(color.begin(),color.end()),color.end());
  rep(i,color.size()) {
    vector<Point> vp;
    rep(j,vec.size()) if( color[i] == find(j) ) vp.push_back(vec[j].p1), vp.push_back(vec[j].p2);
    sort(vp.begin(),vp.end());
    vp.erase(unique(vp.begin(),vp.end()),vp.end());
    assert( vp.size() >= 2 ); // 線分じゃなくて点
    vir = vp[0];
    sort(vp.begin(),vp.end(),comp_dist);
    ret.push_back(Segment(vp.front(),vp.back()));
  }
}

// --作成中--

const double DINF = 1e20;

struct Data {
  int cur;
  double weight;
  bool operator < ( const Data &data ) const { return !equals(weight,data.weight) && weight > data.weight; }
};

void dijkstra(vector<vector<Edge> > &G,vector<Point> &ps){
  int sindex = -1;
  rep(i,(int)ps.size()) if( ps[i] == sp ) { sindex = i; break; }
  assert( sindex != -1 );

  int V = ps.size();
  priority_queue<Data> Q;
  Q.push((Data){sindex,0});
  vector<double> mindist(V,DINF);
  mindist[sindex] = 0;
  while( !Q.empty() ){
    Data data = Q.top(); Q.pop();
    
    if( LT(mindist[data.cur],data.weight) ) continue;

    rep(i,G[data.cur].size()){
      int to = G[data.cur][i].to;
      if( LT(data.weight+G[data.cur][i].cost,mindist[to]) ){
        mindist[to] = data.weight+G[data.cur][i].cost;
        Q.push((Data){to,mindist[to]});
      }
    }

  }

  double maxi = 0;
  rep(i,V) {
    assert( !equals(DINF,mindist[i]) );
    maxi = max(maxi,mindist[i]);
  }

  printf("%.10lf\n",maxi);

}

void compute(){
  vector<Segment> buf = segs;
  //segmentMerge(segs,buf);  

  vector<Point> ps;
  vector<vector<Edge> > G = segmentArrangement(buf,ps);

  dijkstra(G,ps);

}

int main(){
  while( cin >> E, E ){
    segs.resize(E);
    rep(i,E) cin >> segs[i].p1.x >> segs[i].p1.y >> segs[i].p2.x >> segs[i].p2.y;
    cin >> sp.x >> sp.y;
    compute();
  }
  return 0;
}