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

struct punct
{
  long double x,y;
};

punct v[100500];
long double aria;
int main()
{
  freopen("aria.in","r",stdin);
  freopen("aria.out","w",stdout);
  cin>>n;
  for(int i=0;i<n;i++)
    cin>>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;
  cout<<setprecision(6)<<fixed;
  cout<<fabs (aria / 2.0);
}