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

int N;
const long double ERR = 0.0000000001;

struct czworka{
	int x1;
	int y1;
	int x2;
	int y2;
	bool prawo;
	// czy na prawo od wektora P1 -> P2
};

struct wielokat{
	vector<long double> x;
	vector<long double> y;
	// ostatni punkt jest powtorzony
	int S;
};

int ind1, ind2;

vector<czworka> Czar;
vector<czworka> X;

long double xp, yp;
short wp;

bool blisko(long double a, long double b){
	if(abs(a - b) < ERR){
		return true;
	}
	return false;
}

int det(int x1, int y1, int x2, int y2, int x3, int y3){
	int k = x1*y2 + x2*y3 + x3*y1 - y1*x2 - y2*x3 - y3*x1;
	if(k > 0){
		k = 1;
	} else if(k < 0){
		k = -1;
	}
	// det = 1 oznacza, ze P3 jest na LEWO od P1 -> P2
	return k;
}

int det2(long double x1, long double y1, long double x2, long double y2, long double x3, long double y3){
	long double k = x1*y2 + x2*y3 + x3*y1 - y1*x2 - y2*x3 - y3*x1;
	if(blisko(k, 0)){
		return 0;
	} else if(k > 0){
		return 1;
	}
	return -1;
	// det = 1 oznacza, ze P3 jest na LEWO od P1 -> P2
}

bool nowa_prosta(int x1, int y1, int x2, int y2){
	int det1, det2;
	bool czy1 = false;
	bool czy2 = false;
	for(int i = 0; i < N; i++){
		if((i != ind1) && (i != ind2)){
			det1 = det(x1, y1, x2, y2, Czar[i].x1, Czar[i].y1);
			det2 = det(x1, y1, x2, y2, Czar[i].x2, Czar[i].y2);
			if(det1 * det2 == 1){
				if(det1 == 1){
					czy1 = true;
					if(czy2){
						break;
					}
				} else {
					czy2 = true;
					if(czy1){
						break;
					}
				}
			}
		}
	}
	// czy1 jest na lewo
	// czy2 jest na prawo
	czworka nowe;
	nowe.x1 = x1;
	nowe.y1 = y1;
	nowe.x2 = x2;
	nowe.y2 = y2;
	if(!czy1){
		if(!czy2){
			return true;
		}
		nowe.prawo = false;
		X.push_back(nowe);
	}
	if(!czy2){
		nowe.prawo = true;
		X.push_back(nowe);
	}
	return false;
}

void proste_dwie(long double x1, long double y1, long double x2, long double y2, long double x3, long double y3, long double x4, long double y4){
	if(blisko(x1, x2)){
		long double a2 = (y4 - y3)/(x4 - x3);
		long double b2 = y3 - a2 * x3;
		xp = x1;
		yp = a2 * xp + b2;
	} else {
		if(blisko(x3,  x4)){
			long double a1 = (y2 - y1)/(x2 - x1);
			long double b1 = y1 - a1 * x1;
			xp = x3;
			yp = a1 * xp + b1;
		} else {
			long double a1 = (y2 - y1)/(x2 - x1);
			long double b1 = y1 - a1 * x1;
			long double a2 = (y4 - y3)/(x4 - x3);
			long double b2 = y3 - a2 * x3;
			xp = (b1 - b2)/(a2 - a1);
			yp = a1 * xp + b1;
		}
	}
}

wielokat przetnij(wielokat W, long double x1, long double y1, long double x2, long double y2){
	wielokat Q;
	Q.S = 0;
	long double d1, d2;
	d1 = det2(x1, y1, x2, y2, W.x[0], W.y[0]);
	d2 = det2(x1, y1, x2, y2, W.x[1], W.y[1]);
	for(int j = 0; j < W.S - 1; j++){
		if((d1 < 0) && (d2 < 0)){
			// wtedy NIE
		} else if(d1 < 0){
			proste_dwie(x1, y1, x2, y2, W.x[j], W.y[j], W.x[j + 1], W.y[j + 1]);
			Q.x.push_back(xp);
			Q.y.push_back(yp);
			Q.S++;
			if(d2 >= 0){
				Q.x.push_back(W.x[j + 1]);
				Q.y.push_back(W.y[j + 1]);
				Q.S++;
			}
		} else if(d2 < 0){
			if(d1 >= 0){
				proste_dwie(x1, y1, x2, y2, W.x[j], W.y[j], W.x[j + 1], W.y[j + 1]);
				Q.x.push_back(xp);
				Q.y.push_back(yp);
				Q.S++;
			}
		} else {
			Q.x.push_back(W.x[j + 1]);
			Q.y.push_back(W.y[j + 1]);
			Q.S++;
		}
		d1 = d2;
		if(j != W.S - 2){
			d2 = det2(x1, y1, x2, y2, W.x[j + 2], W.y[j + 2]);
		}
	}
	if(Q.S > 0){
		Q.x.push_back(Q.x[0]);
		Q.y.push_back(Q.y[0]);
		Q.S++;
	}
	return Q;
}

long double pole(wielokat W){
	if(W.S < 4){
		return 0;
	}
	long double x1 = W.x[0];
	long double y1 = W.y[0];
	long double P = 0;
	for(int i = 1; i < W.S - 2; i++){
		long double x2 = W.x[i];
		long double y2 = W.y[i];
		long double x3 = W.x[i + 1];
		long double y3 = W.y[i + 1];
		long double PT = abs((x2 - x1) * (y3 - y1) - (y2 - y1) * (x3 - x1));
		P += PT / 2;
	}
	return P;
}

int main(){
	cin >> N;
	czworka nowe;
	for(int i = 0; i < N; i++){
		cin >> nowe.x1 >> nowe.y1 >> nowe.x2 >> nowe.y2;
		Czar.push_back(nowe);
	}
	bool KONIEC = false;
	for(int i = 0; i < N - 1; i++){
		for(int j = i + 1; j < N; j++){
			ind1 = i;
			ind2 = j;
			if((nowa_prosta(Czar[i].x1, Czar[i].y1, Czar[j].x1, Czar[j].y1)) ||
				(nowa_prosta(Czar[i].x2, Czar[i].y2, Czar[j].x1, Czar[j].y1)) ||
				(nowa_prosta(Czar[i].x1, Czar[i].y1, Czar[j].x2, Czar[j].y2)) ||
				(nowa_prosta(Czar[i].x2, Czar[i].y2, Czar[j].x2, Czar[j].y2))){
				KONIEC = true;
				break;
			}
		}
		if(KONIEC) break;
	}
	if(KONIEC){
		cout << setprecision(19) << fixed << (long double)(0.0) << endl;
	} else {
		wielokat W;
		W.x.push_back(-1000);
		W.x.push_back(-1000);
		W.x.push_back(1000);
		W.x.push_back(1000);
		W.x.push_back(-1000);
		W.y.push_back(-1000);
		W.y.push_back(1000);
		W.y.push_back(1000);
		W.y.push_back(-1000);
		W.y.push_back(-1000);
		W.S = 5;
		for(int i = 0; i < X.size(); i++){
			if(X[i].prawo){
				W = przetnij(W, X[i].x1, X[i].y1, X[i].x2, X[i].y2);
			} else {
				W = przetnij(W, X[i].x2, X[i].y2, X[i].x1, X[i].y1);
			}
			if(W.S == 0){
				break;
			}
		}
		if(W.S == 0){
			cout << setprecision(19) << fixed << (long double)(0.0) << endl;
		} else {
			/*cout << "========================== WIERZCHOLKI =============================" << endl;		
			for(int i = 0; i < W.S; i++){
				cout << "(" << W.x[i] << ", " << W.y[i] << ")" << endl; 
			}
			cout << "==========================    KONIEC   =============================" << endl;	*/
			long double P = pole(W);
			//P += ERR/10;
			cout << setprecision(19) << fixed << P << endl;
		}
	}
}