#include<bits/stdc++.h>
using namespace std;
ofstream fout ("aria.out");
ifstream fin ("aria.in");
int n;
struct punct
{
	double x, y;
} v[100010];
double a;
int main ()
{
	fin>>n;
	for (int i = 1; i <= n; i++) fin>>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);
	fout.precision(6);
    fout<<fixed<<max( a / 2.0 , a / -2.0 );
}