#include <fstream>
#include <iomanip>

using namespace std;

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

int n,i,y,x,yi,xi,yt,xt;
double s;

int main()
{
    f>>n;

    f>>xi>>yi;

    xt=xi;yt=yi;

    for (i=2;i<=n;i++)
     {
         f>>x>>y;

         s+=xt*y-yt*x;

         xt=x;yt=y;
     }

     s+=xt*yi-xi*yt;

     if (s<0)
      s=-s;

     s/=2;

     g<<fixed<<setprecision(5)<<s;

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