#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
#define LE  100666
#include <vector>
#define x first
#define y second
#define mp make_pair
double result;
int n,i;
pair<double,double> p1,p2;
double cross(pair<double ,double> p1,pair<double,double> p2)
{
    return p1.x*p2.y-p1.y*p2.x;
}
pair<double ,double> p[LE];
int main()
{
    f>>n;

  for(i=1;i<=n;++i)
    f>>p[i].x>>p[i].y;

   p[n+1]=p[1];

   for(i=1;i<=n;++i)
   {
       p1=p[i];
       p2=mp(p[i+1].x-p[i].x,p[i+1].y-p[i].y);
       result+=(cross(p1,p2)/2.0);
   }
  g<<fixed;
  g<<setprecision(5);

   g<<result<<'\n';




f.close();
g.close();
    return 0;
}