extern crate core;

use std::fmt;
use std::cmp::{Ordering, min, max};
use std::fmt::{Display, Error, Formatter};
use std::f32::MAX;
use std::ops::{Add, Sub, Mul, Div, Neg, Index, IndexMut};
use std::collections::{BTreeMap, VecDeque, BinaryHeap};

fn show<T: Display>(vec: &Vec<T>) {
    if vec.is_empty() {
        println!("[]");
    }else {
        print!("[{}", vec[0]);
        for i in 1 .. vec.len() {
            print!(", {}", vec[i]);
        }
        println!("]");
    }
}
fn show2<T: Display>(vec: &Vec<Vec<T>>) {
    if vec.is_empty() {
        println!("[]");
    }else {
        for l in vec {
            show(l);
        }
    }
}
macro_rules! read_line{
    () => {{
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).ok();
        line
    }};
    (delimiter: ' ') => {
        read_line!().split_whitespace().map(|x|x.to_string()).collect::<Vec<_>>()
    };
    (delimiter: $p:expr) => {
        read_line!().split($p).map(|x|x.to_string()).collect::<Vec<_>>()
    };
    (' ') => {
        read_line!(delimiter: ' ')
    };
    ($delimiter:expr) => {
        read_line!(delimiter: $delimiter)
    };
    (' '; $ty:ty) => {
        read_line!().split_whitespace().map(|x|x.parse::<$ty>().ok().unwrap()).collect::<Vec<$ty>>()
    };
    ($delimiter:expr; $ty:ty) => {
        read_line!($delimiter).into_iter().map(|x|x.parse::<$ty>().ok().unwrap()).collect::<Vec<$ty>>()
    };
}
macro_rules! read_value{
    () => {
        read_line!().trim().parse().ok().unwrap()
    }
}
macro_rules! let_all {
    ($($n:ident:$t:ty),*) => {
        let line = read_line!(delimiter: ' ');
        let mut iter = line.iter();
        $(let $n:$t = iter.next().unwrap().parse().ok().unwrap();)*
    };
}
macro_rules! let_mut_all {
    ($($n:ident:$t:ty),*) => {
        let line = read_line!(delimiter: ' ');
        let mut iter = line.iter();
        $(let mut $n:$t = iter.next().unwrap().parse().ok().unwrap();)*
    };
}

const EPSILON: f64 = 0.000001;
#[derive(Copy, Clone)]
struct Point{
    x: f64, y: f64
}
impl Point {
    fn new(x: f64, y: f64) -> Point {
        Point{x: x, y: y}
    }
    fn distance_from(&self, other: &Point) -> f64 {
        ((self.x - other.x) * (self.x - other.x) + (self.y - other.y) * (self.y - other.y)).sqrt()
    }
}
impl Display for Point {
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        write!(f, "(x: {}, y: {})", self.x, self.y)
    }
}
struct Line {
    a: f64, b: f64, c: f64
}
struct Edge {
    from: Point, to: Point, pub line: Line
}
impl Edge {
    fn new(from: Point, to: Point) -> Edge {
        Edge{from: from, to: to, line: Line::by_two_line(&from, &to)}
    }
    fn cross_point(&self, other: &Self) -> Option<Point> {
        match self.line.cross_point(&other.line) {
            Some(p) if (p.x - self.from.x) * (p.x - self.to.x) <= EPSILON && (p.y - self.from.y) * (p.y - self.to.y) <= EPSILON => Some(p),
            _ => None
        }
    }
    fn is_on_edge(&self, other: &Point) -> bool {
        self.line.is_on_line(other) && (other.x - self.from.x) * (other.x - self.to.x) <= EPSILON && (other.y - self.from.y) * (other.y - self.to.y) <= EPSILON
    }
}
impl Line {
    fn by_two_line(a: &Point, b: &Point) -> Line {
        Line{a: b.y - a.y, b: a.x - b.x, c: a.y * b.x - a.x * b.y}
    }
    fn cross_point(&self, other: &Line) -> Option<Point> {
        if self.is_parallel(other) {
            None
        }else {
            let d = self.a * other.b - self.b * other.a;
            Some(Point{x: (self.b * other.c - self.c * other.b) / d, y: (self.c * other.a - self.a * other.c) / d})
        }
    }
    fn is_parallel(&self, other: &Line) -> bool {
        (self.a * other.b - self.b * other.a).abs() <= EPSILON
    }
    fn is_on_line(&self, other: &Point) -> bool {
        (self.a * other.x + self.b * other.y + self.c).abs() <= EPSILON
    }
}
struct CrossPoint {
    point: Point, to: usize,
}
struct ValueWithKey<K, V> {
    key: K, value: V
}
impl <K, V> ValueWithKey<K, V> {
    fn new(key: K, value: V) -> ValueWithKey<K, V> {
        ValueWithKey{key: key, value: value}
    }
}
impl <K: Copy, V: Copy> Copy for ValueWithKey<K, V> {}
impl <K: Clone, V: Clone> Clone for ValueWithKey<K, V>{
    fn clone(&self) -> Self {
        ValueWithKey{key: self.key.clone(), value: self.value.clone()}
    }
}
impl <K: PartialOrd, V> Ord for ValueWithKey<K, V> {
    fn cmp(&self, other: &Self) -> Ordering {
        other.key.partial_cmp(&self.key).unwrap()
    }
}
impl <K: PartialOrd, V> PartialOrd for ValueWithKey<K, V> {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        other.key.partial_cmp(&self.key)
    }
}
impl <K: PartialOrd, V> PartialEq for ValueWithKey<K, V> {
    fn eq(&self, other: &Self) -> bool {
        other.key.eq(&self.key)
    }
}
impl <K: PartialOrd, V> Eq for ValueWithKey<K, V> {}
#[derive(Copy, Clone)]
struct Path {
    to: usize, cost: f64,
}
fn min_f64(a: f64, b: f64) -> f64 {
    if a > b {
        b
    }else {
        a
    }
}
fn main() {
    loop {
        let_all!(n: usize);
        if n == 0 {
            return
        }
        let mut edges = Vec::with_capacity(n);
        for _ in 0 .. n {
            let_all!(x1: f64, y1: f64, x2: f64, y2: f64);
            edges.push(Edge::new(Point::new(x1, y1), Point::new(x2, y2)));
        }
        let_all!(x: f64, y: f64);
        let start = Point::new(x, y);
        let mut points = vec![Vec::new(); 2 * n + n * (n - 1) / 2 + 1];
        let mut cross = Vec::with_capacity(n);
        for i in 0 .. n {
            let mut cross_points = vec![ValueWithKey{key: 0_f64, value: i * 2}, ValueWithKey{key: edges[i].from.distance_from(&edges[i].to), value: i * 2 + 1}];
            for j in 0 .. n {
                if i != j {
                    if let Some(cp) = edges[i].cross_point(&edges[j]) {
                        if i > j {
                            cross_points.push(ValueWithKey { key: edges[i].from.distance_from(&cp), value: 2 * n + i * (i - 1) / 2 + j });
                        }else {
                            cross_points.push(ValueWithKey { key: edges[i].from.distance_from(&cp), value: 2 * n + j * (j - 1) / 2 + i });
                        }
                    }
                }
            }
            if edges[i].is_on_edge(&start) {
                cross_points.push(ValueWithKey{key: edges[i].from.distance_from(&start), value: 2 * n + n * (n - 1) / 2});
            }
            cross_points.sort();
            for j in 1 .. cross_points.len() {
                //println!("set edge {}, {}, distance {}", cross_points[j].value, cross_points[j - 1].value, cross_points[j - 1].key - cross_points[j].key);
                points[cross_points[j].value].push(Path{to: cross_points[j - 1].value, cost: cross_points[j - 1].key - cross_points[j].key});
                points[cross_points[j - 1].value].push(Path{to: cross_points[j].value, cost: cross_points[j - 1].key - cross_points[j].key});
            }
            cross.push(cross_points);
        }
        let mut memo = vec![std::f64::MAX; points.len()];
        let mut queue = BinaryHeap::new();
        memo[points.len() - 1] = 0_f64;
        queue.push(ValueWithKey{key: 0_f64, value: points.len() - 1});
        while let Some(ValueWithKey{key: distance, value: current}) = queue.pop() {
            if memo[current] == distance {
                for &Path{to: next, cost: length} in &points[current] {
                    if memo[next] > distance + length {
                        memo[next] = distance + length;
                        queue.push(ValueWithKey{key: distance + length, value: next});
                    }
                }
            }
        }
        let mut max = 0_f64;
        for i in 0 .. 2 * n{
            if max < memo[i] {
                max = memo[i];
            }
        }
        for i in 0 .. n {
            for j in 1 .. cross[i].len() - 1 {
                let distance = cross[i][j - 1].key - cross[i][j].key;
                if max < min_f64(memo[cross[i][j - 1].value], memo[cross[i][j].value]) + distance / 2_f64 {
                    max = min_f64(memo[cross[i][j - 1].value], memo[cross[i][j].value]) + distance / 2_f64;
                }
            }
        }
        println!("{:.*}", 10, max);
    }
}


Measured memory_bytes: 37608