#include <fstream>
using namespace std;
ifstream in ("aria.in"); ofstream out ("aria.out");

double abs(double x)
{
  if(x<0)return -x;
  return x;
}

struct point
{
  int x,y;
};

point pts[100003];
int n;
int main()
{
  in >> n;
  for( int i = 0; i < n; i++)
  {
    int x,y;
    in >> x >> y;
    pts[i]=(point){x,y};
  }
  pts[n]=pts[0];
  double ar=0;
  for(int i = 0; i < n; i++)
  {
    ar+=(pts[i].x*pts[i+1].y-pts[i+1].x*pts[i].y)/2;
  }
  out.precision(5);
  out << abs(ar);
}