#include <fstream>
#include <iomanip>
using namespace std;
#define dim 100002

int main(){
	
	ifstream f("aria.in");
	ofstream g("aria.out");

	int n,i;
	double A,x[dim],y[dim];
	f >> n;
	for(i=1;i<=n;i++) f >> x[i] >> y[i];
	x[n+1]=x[1];
	y[n+1]=y[1];
	for(i=1;i<=n;i++){
		A+=(x[i]*y[i+1]-y[i]*x[i+1])/2;
	}
	g << fixed << setprecision(5) <<  A;
}