#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct puncte{double x,y;};
puncte a[100002];
int n,i;
double arie;
int main()
{
   f>>n;
  for (i=1;i<=n;i++)
    f>>a[i].x>>a[i].y;
  a[n+1]=a[1];
   for (i=1;i<=n;i++)
    arie+=(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
   arie/=2;
   g<<fixed;
   g<<setprecision(5);
    if (arie>(-arie)) g<<arie;
    else g<<-arie;
    return 0;
}