#include <fstream>
using namespace std;
long double x[100000],y[100000];
int main(int argc, char *argv[]) {
	ifstream in("aria.in");
	ofstream out("aria.out");
	long long int n;
	long double s=0;
	in>>n;
	for(long long int i=1;i<=n;i++){
		in>>x[i]>>y[i];
	}
	x[0]=x[n];
	y[0]=y[n];
	for(long long int i=1;i<=n;i++){
		s+=((x[i]-x[i-1])*(y[i]+y[i-1])/2);
	}
	s*=(-1);
	out<<s;
}