#include <fstream>
#include <iomanip>
using namespace std;

using punct = pair<long double, long double>;

double det(const punct& a, const punct& b){
	return (a.first * b.second - a.second * b.first)/2; }

int main(){
	ifstream f("aria.in");
	ofstream g("aria.out");
	int n;
	f >> n;
	punct primul, penultimul, ultimul;
	f >> primul.first >> primul.second;
	ultimul = primul;
	double rez = 0;
	for(int i = 1; i < n; ++i){
		penultimul = ultimul;
		f >> ultimul.first >> ultimul.second;
		rez += det(penultimul, ultimul); }
	rez += det(ultimul, primul);
	g << fixed << setprecision(5) << rez;
	return 0; }