#include<bits/stdc++.h>
using namespace std;
int n;
struct punct
{
	double x, y;
} v[100010];
long double a;
int main ()
{
	freopen("aria.in", "r", stdin);
	cin>>n;
	for (int i = 0; i < n; i++)
	{
		cin>>v[i].x>>v[i].y;
	}
	a = 0;
	v[n] = v[0];
	for (int i = 0; i < n; i++)
	{
		a += (v[i].x*v[i+1].y - v[i+1].x*v[i].y)*0.5;
	}
	cout<<setprecision(6)<<fixed;
	cout<<a;
	return 0;
}