#include <iostream>
#include <fstream>
#include <iomanip>
#include <vector>
#include <string>

using namespace std;

ifstream in("aria.in");
ofstream out("aria.out");

struct Point
{
  long double x, y;
};

long double areaWithOrigin(Point a, Point b)
{
  return (a.x * b.y - b.x * a.y) / 2;
}

int main()
{
  int n;
  long double sum = 0, a, b;
  vector<Point> points;

  in >> n;

  for (int i = 0; i < n; i++)
  {
    in >> a >> b;
    points.push_back({a, b});
  };

  points.push_back(points[0]);

  for (int i = 0; i < n; i++)
  {
    sum += areaWithOrigin(points[i], points[i + 1]);
  }

  out << setprecision(6) << fixed << sum << "\n";
  return 0;
}