#include <vector>
#include <fstream>
#include <iostream>
#include <iomanip>


std::ifstream fin("aria.in");
std::ofstream fout("aria.out");

int N;
std::vector<std::pair<double, double> > points;

double area(std::pair<double, double> p1, std::pair<double, double> p2) {
	return 1/2 * (p1.first * p2.second - p2.first * p1.second);
}



int main()
{
	fin >> N;
	for (int i = 0; i < N; i++) {
		double x, y;
		fin >> x >> y;
		points.push_back(std::make_pair(x, y));
	}

	double sum = 0;

	for (int i = 0; i < N; i++) {
		sum += area(points[i], points[(i + 1) % N]);
	}


	fout << std::fixed << std::setprecision(6) << sum << '\n';
    return 0;
}