#include <fstream>
#include <iomanip>

using namespace std;

int main() {
	int N;
	int* x;
	int* y;
	long double A;

	// input
	ifstream f("aria.in");
	f >> N;
	x = new int[N];
	y = new int[N];
	for (int i = 0; i < N; i++) {
		f >> x[i] >> y[i];
	}
	f.close();

	// compute
	A = 0.0;
	for (int i = 0; i < N; i++) {
		A += static_cast<long double>((x[i] * y[(i + 1) % N] - x[(i + 1) % N] * y[i]) / 2.0);
	}
	
	// output
	ofstream g("aria.out");
	g << fixed << setprecision(5) << A << "\n";
	g.close();

	// free memory
	delete[] x;
	delete[] y;

	return 0;
}