#include<bits/stdc++.h>
using namespace std;
int n;
struct punct
{
	double x, y;
} v[100010];
double a;
int main ()
{
	freopen("aria.in", "r", stdin);
	freopen("aria.out", "w", stdout);
	cin>>n;
	for (int i = 1; i <= n; i++) cin>>v[i].x>>v[i].y;
	v[n+1] = v[1];
	for (int i = 1; i <= n; i++) a += (v[i].x*v[i+1].y - v[i+1].x*v[i].y);
	cout<<setprecision(6)<<fixed;
	cout<<max(a/2.0, a/-2.0);
	return 0;
}