#include <fstream>
#include <iomanip>
using namespace std;
ifstream cin( "aria.in" );
ofstream cout( "aria.out" );

int n, s;
struct punct
{
	double x, y;
}v[ 100010 ];
int main()
{
	int i;
	cin >> n;
	for ( i = 1; i <= n; i++ )
		cin >> v[ i ].x >> v[ i ].y;
	v[ n + 1 ] = v[ 1 ];
	for ( i = 1; i <= n; i++ )
		s += ( v[ i ].x * v[ i + 1 ].y - v[ i ].y * v[ i + 1 ].x );
	if ( s < 0 ) s *= -1;
	cout << setprecision( 6 ) << s / 2.0;
}