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

template<class T>
struct Point {
	typedef Point P;
	T x, y;
	explicit Point(T x=0, T y=0) : x(x), y(y) {}
	P operator+(P p) const { return P(x+p.x, y+p.y); }
	P operator-(P p) const { return P(x-p.x, y-p.y); }
	P operator*(T d) const { return P(x*d, y*d); }
	P operator/(T d) const { return P(x/d, y/d); }
	T dist2() const { return x*x + y*y; }
	double dist() const { return sqrt((double)dist2()); }
	T dot(P p) const { return x*p.x + y*p.y; }
	P unit() const { return *this/dist(); } // makes dist()=1
};

typedef Point<long double> P;

int n;
vector<pair<P, P>> segments;
P startp, endp;
vector<long double> values;

P point(int i) {
    if(i == -1) return startp;
    if(i == n) return endp;
    return segments[i].first * (1 - values[i]) + segments[i].second * values[i];
}

long double total_dist() {
    long double result = 0;
    for(int i = 0; i <= n; i++) result += (point(i) - point(i-1)).dist();
    return result;
}

void step(long double multiplier) {
    long double result = total_dist();

    vector<long double> grads(n);
    for(int i = 0; i < n; i++) {
        P cur = point(i), prev = point(i-1), next = point(i + 1);
        
        long double grad1 = (prev - cur).unit().dot(segments[i].second - segments[i].first);
        long double grad2 = (next - cur).unit().dot(segments[i].second - segments[i].first);
        grads[i] = grad1 + grad2;
    }

    for(int i = 0; i < n; i++) {
        values[i] += grads[i] * multiplier;
        values[i] = clamp(values[i], 0.0l, 1.0l);
    }
}

int main() {
	cin >> n;
	segments.resize(n);
    values.resize(n);
	cin >> startp.x >> startp.y;
	long double multiplier = 0;
    for(int i = 0; i < n; i++) {
		cin >> segments[i].first.x >> segments[i].first.y >> segments[i].second.x >> segments[i].second.y;
        multiplier += (segments[i].second - segments[i].first).dist();
    }
	cin >> endp.x >> endp.y;

	for(int i = 0; i < 1000000; i++) step(0.1 / multiplier);

    cout << setprecision(15) << fixed << total_dist() << "\n";
}