#include <fstream>
#include <algorithm>
#include <cstring>
#include <iomanip>
#include <vector>
using namespace std;
ifstream fin("camera.in");
ofstream fout("camera.out");
const double eps = 1e-6;
struct Point {
double x, y;
Point() {}
Point(double x, double y) {
this->x = x;
this->y = y;
}
};
int n;
vector< pair<Point, Point> > edges;
vector<Point> polygon;
inline double det(const Point &a, const Point &b, const Point &c) {
return (b.x - a.x)*(c.y - a.y) - (b.y - a.y)*(c.x - a.x);
}
inline int sign(double x) {
if (x > -eps && x < eps)
return 0;
if (x > 0)
return 1;
return -1;
}
Point intersect(const Point &p1, const Point &p2, const Point &p3, const Point &p4) {
double a1 = p2.y - p1.y;
double b1 = p1.x - p2.x;
double c1 = p2.x*p1.y - p1.x*p2.y;
double a2 = p4.y - p3.y;
double b2 = p3.x - p4.x;
double c2 = p4.x*p3.y - p3.x*p4.y;
double div = a1*b2 - a2*b1;
double x = (b1*c2 - b2*c1) / div;
double y = (a2*c1 - a1*c2) / div;
return Point(x, y);
}
vector<Point> aux;
inline void cutPolygon(const Point &a, const Point &b) {
aux.clear();
for (unsigned int i = 0; i + 1 < polygon.size(); ++i) {
if (sign(det(polygon[i], a, b)) * sign(det(polygon[i + 1], a, b)) < 0) {
Point temp = intersect(polygon[i], polygon[i + 1], a, b);
if (sign(det(polygon[i], a, b)) > 0) {
aux.push_back(polygon[i]);
aux.push_back(temp);
}
else {
aux.push_back(temp);
aux.push_back(polygon[i + 1]);
}
continue;
}
if (sign(det(polygon[i], a, b)) >= 0)
aux.push_back(polygon[i]);
}
aux.push_back(aux[0]);
polygon = aux;
}
int main() {
fin >> n;
Point p0, prev;
fin >> p0.x >> p0.y;
prev = p0;
double maxX = p0.x, minX = p0.x;
double maxY = p0.y, minY = p0.y;
for (int i = 2; i <= n; ++i) {
Point curr;
fin >> curr.x >> curr.y;
maxX = max(maxX, curr.x);
minX = min(minX, curr.x);
maxY = max(maxY, curr.y);
minY = min(minY, curr.y);
edges.push_back(make_pair(prev, curr));
prev = curr;
}
edges.push_back(make_pair(prev, p0));
double aria = 0;
for (unsigned int i = 0; i < edges.size(); ++i)
aria += det(Point(0, 0), edges[i].first, edges[i].second);
if (aria < 0) {
for (unsigned int i = 0; i < edges.size(); ++i)
swap(edges[i].first, edges[i].second);
for (int i = 0, j = edges.size() - 1; i < j; ++i, --j)
swap(edges[i], edges[j]);
}
polygon.push_back(Point(minX, minY));
polygon.push_back(Point(maxX, minY));
polygon.push_back(Point(maxX, maxY));
polygon.push_back(Point(minX, maxY));
polygon.push_back(Point(minX, minY));
for (unsigned int i = 0; i < edges.size(); ++i) {
cutPolygon(edges[i].first, edges[i].second);
}
aria = 0;
for (unsigned int i = 0; i + 1 < polygon.size(); ++i)
aria += det(Point(0, 0), polygon[i], polygon[i + 1]);
aria /= 2.0;
if (aria < 0)
aria = -aria;
fout << setprecision(4) << fixed << aria << '\n';
return 0;
}
//Trust me, I'm the Doctor!