#include <fstream>
#include <iomanip>
using namespace std;
ifstream f ("aria.in");
ofstream g ("aria.out");
int n;
double s;
struct
{
      double x,y;
}v[100003];
int main()
{
      f>>n;
      for(int i=1;i<=n;++i) f>>v[i].x>>v[i].y;
      v[n+1]=v[1];
      for(int i=1;i<=n;++i) s=s+v[i].x*v[i+1].y-v[i+1].x*v[i].y;
      if(s<0) s=-s;
      s/=2.0;
      g<<fixed<<setprecision(6)<<s;
      return 0;
}