#include <fstream>
#include <iomanip>

using namespace std;

const int NMAX = 100005;
int n;
pair <double, double> v[NMAX];
double ans = 0;

int main()
{
	ifstream fin("aria.in");
	ofstream fout("aria.out");
	fin >> n;
	for (int i = 1;i <= n;++i)
		fin >> v[i].first >> v[i].second;
	v[n + 1] = v[1];
	for (int i = 1;i <= n;++i)
		ans += v[i].first * v[i + 1].second - v[i + 1].first * v[i].second;
	fout << setprecision(5) << fixed;
	fout << ans / 2.0 << "\n";
	fin.close();
	fout.close();
	return 0;
}