#include <stdio.h>
using namespace std;

int n, s;
struct punct
{
	double x, y;
}v[ 100010 ];
int main()
{
	freopen( "aria.in","r",stdin );
	freopen( "aria.out","w",stdout );
	int i;
	scanf( "%d",&n );
	for ( i = 1; i <= n; i++ )
	scanf( "%lf%lf",&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;
	printf( "%lf",s / 2.0 );
}