#include <iostream>
#include <fstream>
#include <iomanip>
#include <math.h>
using namespace std;
ifstream f( "aria.in" );
ofstream g( "aria.out" );
int N,i,j;
long double Aria;
struct Punct { long double x,y; }V[100001];
int main () {
	f>>N;
	for ( i=1;i<=N;i++ ) f>>V[i].x>>V[i].y;
	V[N+1]=V[1];
	for ( i=1;i<=N;i++ ) Aria+=( V[i].x*V[i+1].y-V[i].y*V[i+1].x );
	g<<setprecision(6)<<fabs(Aria/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));
}*/