#include<bits/stdc++.h>
using namespace std;
int n;
struct ustalosti
{
	double x,y;
};
ustalosti a[100100];
double rez;
int main()
{
	ifstream cin("aria.in");
	ofstream cout("aria.out");
	cin >> n;
	for (int i = 0; i < n; i++)
		cin >> a[i].x >> a[i].y;
	a[n].x = a[0].x;
	a[n].y = a[0].y;
	for (int i = 0; i <= n; i++)
	{
		rez += (a[i].x*a[i+1].y - a[i+1].x * a[i].y); 
	}
	cout << setprecision(10) << fixed << rez/2.0;
}