#include <iostream>
#include <fstream>
#include <iomanip>

#define NMAX 100005

using namespace std;

ifstream in ( "aria.in" );
ofstream out ( "aria.out" );

pair < double , double > Points[NMAX];
int N ;
double Answer ;

void Real ( double &X ){
   (X>0?X=X:X=-X);
}

int main ( void ){
   int i  , j ;
   in >> N ;
   for (  i = 1 ; i <= N ; ++i )
      in >> Points[i].first >> Points[i].second;
    Points[N+1] = Points[1];
   for ( i = 1 ; i <= N ; ++i )
   Answer += Points[i].first*Points[i+1].second - Points[i].second*Points[i+1].first;
   Answer/= 2;
   Real(Answer);
   out << setprecision(6) << fixed << Answer << "\n";
   return 0 ;
}