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

struct punct
{
  double x,y;
};

double calc(punct p1, punct p2)
{
  return p1.x*p2.y- p2.x*p1.y;
}
double aria;
int main()
{
  freopen("aria.in","r",stdin);
  freopen("aria.out","w",stdout);
  cin>>n;
  punct p,in;
  double xx,yy;
  scanf("%lf %lf",&xx,&yy);
  p={xx,yy};
  in=p;
  for(int i=1;i<n;i++)
  {
    double xx,yy;
    scanf("%lf %lf",&xx,&yy);
    punct last={xx,yy};
    aria+=calc(p,last);
    p=last;
  }
  aria+=calc(p,in);
  printf ("%lf", fabs (aria / 2.0));
}