#include<fstream>
#define dim 100007


using namespace std;


ifstream f("aria.in");
ofstream g("aria.out");
double ans;
double x[dim],y[dim];
int main (){
	
	int n,i;
	f>>n;
	for(i=1;i<=n;++i){
		f>>x[i]>>y[i];
	}
	for(i=1;i<n;++i){
		ans+=(double)(x[i]*y[i+1]-x[i+1]*y[i]);
	}
	ans+=(double)(x[i]*y[1]-x[1]*y[i]);
	g<<ans/2.0;
	return 0;
}