#include<fstream>
using namespace std;
int n;
double a, x[100005], y[100005];
int main ()
{
	freopen("aria.in", "r", stdin);
	freopen("aria.out", "w", stdout);
	cin>>n;
	for (int i = 0; i < n; i++)
	{
		cin>>x[i]>>y[i];
	}
	a = 0;
	x[n] = x[0];
	y[n] = y[0];
	for (int i = 0; i < n; i++)
	{
		a += (x[i]*y[i+1] - x[i+1]*y[i])/2;
	}
	cout<<setprecision(6)<<fixed;
	cout<<a;
	return 0;
}