#include <fstream>
#include <iomanip>

using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

const int MAXN = 100005;
int n;
long double x[MAXN], y[MAXN];

int main()
{
	cin >> n;
	long double suma = 0;
	for (int i = 1; i <= n; i++)
		cin >> x[i] >> y[i];
	x[n + 1] = x[1];
	y[n + 1] = y[1];
	for (int i = 1; i <= n; i++)
		suma += x[i] * y[i + 1] - x[i + 1] * y[i];
	cout << setprecision(5) << fixed << suma / 2;
	return 0;
}