#include <fstream>
using namespace std;
double A = 0,a,b,c,d;
pair<double, double> p;
int main(){
	ifstream f("aria.in");
	ofstream of("aria.out");
	int N; f >> N;
	f >> p.first >> p.second;
	c = p.first; d = p.second;
	if (N > 1){
		for (int i = 1; i < N; ++i){
			f >> a >> b;
			A += p.first*b - a*p.second;
			p.first = a; p.second = b;
		}
		A += p.first*d - p.second*c;
		of << A / 2;
	}
	else of << "0";
}