#include <fstream>
#include <cmath>
#include <iomanip>

struct point {
	double x, y;
};

point vec[100005];

int main() {
	std::ifstream fin("aria.in");
	std::ofstream fout("aria.out");
	int nrn;
	double ans = 0;
	fin >> nrn;
	for (int index = 0; index < nrn; index++) {
		fin >> vec[index].x >> vec[index].y;
	}
	vec[nrn] = vec[0];
	vec[nrn + 1] = vec[1];
	for (int index = 1; index <= nrn; index++) {
		ans += vec[index].x * (vec[index + 1].y - vec[index - 1].y);
	}
	fout << std::fixed << std::setprecision(6) << ans / 2;
}