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

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

const int NMAX = 1e5 + 5;

pair<long double, long double> v[NMAX];
int main() {
	int n;
	fin >> n;

	double ans = 0;
	fin >> v[1].first >> v[1].second;
	for (int i = 2; i <= n; i++) {
		fin >> v[i].first >> v[i].second;

		ans += (v[i - 1].first * v[i].second - v[i].first * v[i - 1].second);
	}

	ans += (v[n].first * v[1].second - v[1].first * v[n].second);
	ans /= 2.0;

	fout << fixed << setprecision(10) << ans;
	return 0;
}