#include <fstream>
#include <iomanip>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int n;
double r;
struct point
{ double x,y;
}p[100002];
int main()
{ in>>n;
  for(int i=1;i<=n;i++)
    in>>p[i].x>>p[i].y;
  p[n+1]=p[1];
  for(int i=1;i<=n;i++)
    r+=p[i].x*p[i+1].y-p[i+1].x*p[i].y;
  r/=2.0;
  out<<setprecision(6)<<fixed<<r;
  in.close();
  out.close();
  return 0;
}