#include <iostream>
#include <fstream>
#include <iomanip>
#include <math.h>
using namespace std;
ifstream f( "aria.in" );
ofstream g( "aria.out" );
int N,i,j;
double Aria;
struct Punct { double x,y; }V[100001];
int main () {
	f>>N;
	for ( i=1;i<=N;i++ ) f>>V[i].x>>V[i].y;
	V[N]=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(5)<<fabs(Aria);
}