#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

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

int N ;
vector < pair < double , double > > LISTA ;

void Citire ()
{
    fin >> N ;
    double x , y ;
    for ( int i = 1 ; i <= N ; ++ i )
           fin >> x >> y , LISTA.push_back ( make_pair ( x , y ) ) ;

}

double aria ;
int main()
{
    Citire () ;
    LISTA.push_back ( LISTA [0] ) ;

    for ( unsigned int i = 0 ; i < LISTA.size () ; ++ i )
    {
        aria += LISTA[i].first * LISTA[ i + 1 ].second - LISTA [ i + 1 ].first * LISTA [i].second ;
    }
    if ( aria < 0 )
         aria *= -1;

    fout << aria / 2.0 ;
    return 0;
}