#include <iostream>
#include <fstream>
#include <math.h>
using namespace std;
int n;

struct punct
{
  double x,y;
};
double aria;
int main()
{
  freopen("aria.in","r",stdin);
  freopen("aria.out","w",stdout);
  cin>>n;
  punct p,in;
  scanf("%lf %lf",&p.x,&p.y);
  in=p;
  for(int i=1;i<n;i++)
  {
    punct l;
    scanf("%lf %lf",&l.x,&l.y);
    aria+=p.x*l.y- l.x*p.y;
    p=l;
  }
  aria+=p.x*in.y- in.x*p.y;
  printf ("%lf", fabs (aria / 2.0));
}