#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

ifstream fin ("aria.in");
ofstream fout ("aria.out");

int n;
vector<pair<double,double> > points;

int main()
{
	fin>>n;
	double area = 0;
	for (int i = 1; i <= n; ++i)
	{
		double x,y;
		fin>>x>>y;
		points.push_back(make_pair(x,y));
	}
	for (int i = 0; i <= n-2; i++)
	{
		area += 0.5*(points[i].first * points[i+1].second - points[i+1].first * points[i].second);
	}
	area += 0.5*(points[n-1].first * points[0].second - points[0].first * points[n-1].second);
	fout<<area;
}