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

struct punct
{
  double x,y;
};

punct v[100500];
double aria;
int main()
{
  freopen("aria.in","r",stdin);
  freopen("aria.out","w",stdout);
  cin>>n;
  for(int i=0;i<n;i++)
    scanf("%lf %lf",&v[i].x,&v[i].y);
  v[n]=v[0];
  for(int i=0;i<=n;i++)
    aria+=v[i].x*v[i+1].y- v[i+1].x*v[i].y;
  printf ("%lf", fabs (aria / 2.0));
}