#include<fstream>
#include<iomanip>
#define DIM 100100
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i;
double S, A,x[DIM],y[DIM];
int main ()
{
	f>>n;
	for(i=1;i<=n;++i)
	{
		f>>x[i]>>y[i];
	}
	for(i=1;i<n;++i)
	{
		S=x[i]*y[i+1]-x[i+1]*y[i];
		A+=S;
	}
	A+=(x[n]*y[1]-x[1]*y[n]);
	if(A<0)
	A=-A;
	A/=2.0;
	g<<fixed<<setprecision(6)<<A;
	return 0;
}