#include <fstream>
#include <iomanip>

const char IN [ ] = "aria.in" ;
const char OUT [ ] = "aria.out" ;
const int MAX = 100014 ;

using namespace std;

ifstream fin ( IN ) ;
ofstream fout ( OUT ) ;

struct puncte {
    double x , y ;
};
puncte q [ MAX ] ;
double sol ;
int main( )
{
    int n ;
    fin >> n ;
    for ( int i = 1 ; i <= n ; ++ i )
        fin >> q [ i ].x >> q[ i ].y ;
    q [ 0 ] = q [ n ] ;
    for ( int i = 0 ; i < n ; ++ i )
        sol = sol + q[i].x*q[i+1].y - q[i+1].x*q[i].y;
    fout << fixed << setprecision ( 6 ) << sol / 2.0 << '\n';
    return 0;
}