#include <fstream>
#include<iomanip>
#define NMAX 100005
using namespace std;
 
ifstream f("aria.in");
ofstream g("aria.out");
 
long n;
double x1,y1,x2,y2,s,xf,yf;
 
int main()
{
     f>>n;
     for (long i=1;i<=n-1;i++)
      {
        f>>x1>>y1>>x2>>y2;
        s+=x1*y2-x2*y1;
          if(i==1)
        {xf=x1;yf=y1;}
          if(i==n-1)
        s+=x2*yf-xf*y2;
 
        }
 
 
      g<<setprecision(4)<<s/2;