#include <iostream>
#include <fstream>
#include <cmath>

using namespace std;

typedef struct {
	double x, y;
} punct;

punct num[100001];

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

	int n;
	fin >> n;

	for (int i = 0; i < n; i++)
		fin >> num[i].x >> num[i].y;
	num[n] = num[0];

	double suma = 0;
	for (int i = 0; i < n; i++)
		suma += num[i].x * num[i + 1].y - num[i + 1].x * num[i].y;

	fout << fixed << fabs(suma / 2.0) << endl;
}