#include <fstream>

using namespace std;

int main() {
	ifstream in("aria.in");
	ofstream out("aria.out");

	int n;
	in >> n;

	double aria = 0;
	double x[n+2], y[n+2];
	for(int i=0; i<n; ++i)
		in >> x[i] >> y[i];

	x[n] = x[0];
	y[n] = y[0];

	for(int i=0; i<n; ++i)
		aria += (x[i] * y[i+1] - x[i+1] * y[i]);

    if (aria >= 0)
        out << aria / 2;
    else
        out << -aria / 2;
	return 0;
}