// http://infoarena.ro/problema/infasuratoare
#include <fstream>
#include <iomanip>
#include <cmath>
#include <algorithm>
#include <vector>
using namespace std;
#define x first
#define y second
ifstream in("infasuratoare.in");
ofstream out("infasuratoare.out");
const int oo = 0x3f3f3f3f;
int nrPoints;
pair<double,double> minPoint = make_pair(oo,oo);
vector< pair<double,double> > points,convexHull;
double dist(pair<double,double> one,pair<double,double> two);
bool leftTurn(pair<double,double> one,pair<double,double> two,pair<double,double> three);
double determinant(pair<double,double> one,pair<double,double> two,pair<double,double> three);
class compare {
public:
bool operator() (pair<double,double> one,pair<double,double> two) {
return (leftTurn(minPoint,one,two));
}
};
int main() {
in >> nrPoints;
points.reserve(nrPoints);
pair<double,double> point;
for(int i=1;i<=nrPoints;i++) {
in >> point.x >> point.y;
// retinem punctul cu abscisa minima,
// iar in caz de egalitate cu ordonata minima
minPoint = min(minPoint,point);
points.push_back(point);
}
in.close();
// stergem punctul minim
points.erase(find(points.begin(),points.end(),minPoint));
// sortam punctele trigonometric dupa punctul de minim
sort(points.begin(),points.end(),compare());
// punctul de minim este sigur pe infasuratoarea convexa
convexHull.push_back(minPoint);
// la fel si urmatorul
convexHull.push_back(points.front());
for(int i=1;i<points.size();i++) {
// cat timp ultimele doua puncte de pe infasuratoare
// si cel selectat fac o intoarcere spre dreapta,
// sterg ultimul punct de pe infasuratoarea convexa
while(!leftTurn(convexHull[convexHull.size()-2],convexHull[convexHull.size()-1],points[i]))
convexHull.pop_back();
convexHull.push_back(points[i]);
}
out << convexHull.size() << fixed << setprecision(6) << "\n";
for(unsigned int i=0;i<convexHull.size();i++)
out << convexHull[i].x << " " << convexHull[i].y << "\n";
out.close();
return (0);
}
bool leftTurn(pair<double,double> one,pair<double,double> two,pair<double,double> three) {
return (determinant(one,two,three) > 0);
}
double determinant(pair<double,double> one,pair<double,double> two,pair<double,double> three) {
return (one.x * two.y + three.x * one.y + two.x * three.y
- three.x * two.y - one.x * three.y - two.x * one.y);
}