#include <fstream>
#include <iomanip> 

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

long double x[100005], y[100005];

int main() {
	
	int i, n;

	f >> n;
	for (i = 1; i <= n; ++i) {
		f >> x[i] >> y[i];
	}
	x[n + 1] = x[1];
	y[n + 1] = y[1];

	long double aria = 0;
	for (i = 1; i <= n; ++i) {
		aria += (x[i] * y[i + 1] - x[i + 1] * y[i]) / 2;
	}

	if (aria < 0)
		aria = -aria;

	g << fixed << setprecision(6) <<  aria << '\n';

	return 0;
}