#include <stdio.h>
#include <math.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 );
	printf( "%lf",fabs( s / 2.0 ) );
}
/*
# include <cassert>
# include <cstdio>
# include <cmath>
  
const char *FIN = "aria.in", *FOU = "aria.out";
const int MAX = 100005;
  
struct punct {
    double x, y;
} vec[MAX];
  
int N;
double sol;
  
int main (void) {
    assert (freopen (FIN, "r", stdin));
    assert (freopen (FOU, "w", stdout));
  
    assert (scanf ("%d", &N) == 1);
    assert (1 <= N && N <= 100000);
    for (int i = 0; i < N; ++i) {
        assert (scanf ("%lf %lf", &vec[i].x, &vec[i].y) == 2);
        assert (-10000000 <= vec[i].x && vec[i].x <= 10000000);
        assert (-10000000 <= vec[i].y && vec[i].y <= 10000000);
    }
    vec[N] = vec[0];
    for (int i = 0; i < N; ++i)
        sol += (vec[i].x * vec[i + 1].y - vec[i + 1].x * vec[i].y);
    printf ("%lf", fabs (sol / 2.0));
}
*/