#include<bits/stdc++.h>
#define MAX 100000
#define inf 1<<29
#define linf 1e18
#define eps (1e-8)
#define mod 1000000007
#define pi M_PI
#define f first
#define s second
#define mp make_pair
#define pb push_back
#define all(a) (a).begin(),(a).end()
#define pd(a) printf("%.10f\n",(double)(a))
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(a)-1;(b)<=i;i--)
#define equals(a,b) (fabs((a)-(b))<eps)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<double,int> pdi;
typedef pair<int,double> pid;
typedef vector<int> vi;
typedef vector<pii> vpi;
const int dx[8]={1,0,-1,0,1,1,-1,-1};
const int dy[8]={0,1,0,-1,1,-1,1,-1};

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 k){ return Point(x*k,y*k);}
  Point operator/(double k){ return Point(x/k,y/k);}
  bool operator<(Point p)const{ return (x!=p.x ? x-p.x<-eps : y-p.y<-eps);}
  bool operator==(Point p)const{ return fabs(x-p.x)<eps && fabs(y-p.y)<eps;}
 
  double abs(){ return sqrt(norm());}
  double norm(){ return (x*x+y*y);}
};
typedef Point Vector;
typedef vector<Point> Polygon;
 
class Segment{
public:
  Point p1,p2;
  Segment(Point p1=Point(),Point p2=Point()):p1(p1),p2(p2){}
};
typedef Segment Line;
 
double norm(Vector a){ return (a.x*a.x+a.y*a.y);}
double abs(Vector a){ return sqrt(norm(a));}
double dot(Vector a,Vector b){ return (a.x*b.x+a.y*b.y);}
double cross(Vector a,Vector b){ return (a.x*b.y-a.y*b.x);}
 
int ccw(Point p0,Point p1,Point p2){
  Vector a=p1-p0;
  Vector b=p2-p0;
  if(cross(a,b)>eps)return 1;
  if(cross(a,b)<-eps)return -1;
  if(dot(a,b)<-eps)return 2;
  if(a.norm()<b.norm())return -2;
  return 0;
}
 
bool intersect(Point p1,Point p2,Point p3,Point p4){
  return (ccw(p1,p2,p3)*ccw(p1,p2,p4)<=0 &&
          ccw(p3,p4,p1)*ccw(p3,p4,p2)<=0);
}
 
bool intersect(Segment s1,Segment s2){
  return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}
 
Point getCrossPointLL(Line a,Line b){
  double A=cross(a.p2-a.p1,b.p2-b.p1);
  double B=cross(a.p2-a.p1,a.p2-b.p1);
  if(abs(A)<eps || abs(B)<eps)return b.p1;
  return b.p1+(b.p2-b.p1)*(B/A);
}

typedef vector<vector<int> > Graph;
 
Graph SegmentArrangement(vector<Segment> v,vector<Point> &ps){
  for(int i=0;i<v.size();i++){
    ps.push_back(v[i].p1);
    ps.push_back(v[i].p2);
    for(int j=i+1;j<v.size();j++){
      if(intersect(v[i],v[j]))ps.push_back(getCrossPointLL(v[i],v[j]));
    }
  }
  sort(ps.begin(),ps.end());
  ps.erase(unique(ps.begin(),ps.end()),ps.end());
  Graph g(ps.size());
  for(int i=0;i<v.size();i++){
    vector<pair<double,int> > list;
    for(int j=0;j<ps.size();j++)
      if(ccw(v[i].p1,v[i].p2,ps[j])==0)
        list.push_back(mp(norm(v[i].p1-ps[j]),j));
    sort(list.begin(),list.end());
    for(int j=0;j<list.size()-1;j++){
      int a=list[j].s,b=list[j+1].s;
      g[a].push_back(b);
      g[b].push_back(a);
    }
  }
  return g;
}

Graph g;
vector<Point> vp;
int n;
vector<Segment> vs;
Point s;
int start;
double d[MAX];

void init(){
  vs.clear();
  vp.clear();
  g.clear();
}

void dijkstra(){
  FOR(i,0,MAX)d[i]=linf;
  priority_queue<pdi,vector<pdi>,greater<pdi> > pq;
  pq.push(mp(0.0,start));
  d[start]=0.0;

  while(pq.size()){
    pdi u=pq.top();
    pq.pop();

    if(d[u.s]-u.f<-eps)continue;

    FOR(i,0,g[u.s].size()){
      int next=g[u.s][i];
      double cost=abs(vp[u.s]-vp[next]);
      if(u.f+cost-d[next]<-eps){
        d[next]=u.f+cost;
        pq.push(mp(d[next],next));
      }
    }
  }
}

double solve(){
  double res=0;
  vp.pb(s);
  g=SegmentArrangement(vs,vp);
  FOR(i,0,vp.size())if(s==vp[i])start=i;
  dijkstra();
  FOR(i,0,vp.size()){
    if(d[i]!=linf){
      res=max(res,d[i]);
      FOR(j,0,g[i].size()){
        int next=g[i][j];
        double cost=abs(vp[next]-vp[i]);
        res=max(res,min(d[i],d[next])+(cost-abs(d[i]-d[next]))/2.0);
      }
    }
  }
  return res;
}

int main()
{
  while(1){
    cin>>n;
    if(n==0)break;
    init();
    FOR(i,0,n){
      double a,b,c,d;
      cin>>a>>b>>c>>d;
      vs.pb(Segment(Point(a,b),Point(c,d)));
    }
    cin>>s.x>>s.y;
    pd(solve());
  }
  return 0;
}

Measured memory_bytes: 9472