#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <fstream>
using namespace std;
 
const string file = "aria";
 
const string infile = file + ".in";
const string outfile = file + ".out";

struct Point2D
{
	double x;
	double y;
};


int N;
vector<Point2D> points;




double Area(Point2D& p1, Point2D& p2, Point2D& p3)
{
	double result = 0;


	result += p1.x * p2.y + p2.x * p3.y + p3.x * p1.y;
	result -= p3.x * p2.y + p2.x * p1.y + p1.x * p3.y;


	return result/2;
}



int main()
{
	fstream fin(infile.c_str(), ios::in);

	fin >> N;
	points.resize(N);

	for(int i = 0; i < N; i++)
	{
		Point2D p;
		fin >> p.x
			>> p.y;
		points[i] = p;
	}

	fin.close();

	double sol = 0;

	Point2D ref;
	ref.x = 0;
	ref.y = 0;

	for(int i = 0; i < N; i++)
	{
		sol += Area(ref, points[i], points[(i+1) % N]);
	}

	fstream fout(outfile.c_str(), ios::out);

	fout << sol << "\n";

	fout.close();
}