#include <fstream>
#include <iomanip>
using namespace std;
pair<double, double> p[100001];
double A = 0;
int main(){
	ifstream f("aria.in");
	ofstream of("aria.out");
	int N;
	f >> N;
	for (int i = 0; i < N; ++i){
		f >> p[i].first >> p[i].second;
	}
	p[N].first = p[0].first;
	p[N].second = p[0].second;
	for (int i = 0; i < N; ++i)
		A += p[i].first*p[i + 1].second - p[i + 1].first*p[i].second;
	of << setprecision(5)<<A/2.0;
}