#include<iostream>
#include<fstream>
using namespace std;

struct punct {
	long double x;
	long double y;
};

int main()
{
	int N;
	long double aria = 0.0f;
	punct *v = new punct[100000];
	ifstream f("aria.in");
	ofstream g("aria.out");
	f >> N;
	for (int i = 0; i < N; i++) {
		f >> v[i].x >> v[i].y;
	}

	int k = 0;
	while (k < N - 1) {
			aria = aria + (v[k].x * v[k + 1].y - v[k + 1].x * v[k].y);
			k++;
	}

	aria = aria + (v[N-1].x * v[0].y - v[0].x * v[N-1].y);
	aria = aria / 2.0;

	g << aria;

	f.close();
	g.close();
	return 0;
}