#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

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

const int NMAX = 1e5 + 10;

struct Coord{
  double x;
  double y;
};

int n;
double res;
Coord v[1 + NMAX];

int main()
{
  in >> n;
  for(int i = 1; i <= n; i++)
    in >> v[i].x >> v[i].y;

  v[n + 1] = v[1];
  for(int i = 1; i <= n; i++)
    res += (v[i].x * v[i + 1].y - v[i + 1].x * v[i].y);

  out << fixed << setprecision(6);
  out << res / 2 << '\n';

  out.close();
  return 0;
}