/*
	http://www.infoarena.ro/problema/aria
*/

#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

struct point { int x, y; };

point a[100001];
int N;
long double solutia;

void read()
{
	ifstream fin("aria.in");
	fin >> N;
	for (int i = 0; i < N; ++i)
	{
		fin >> a[i].x >> a[i].y;
	}
	fin.close();
	a[N] = a[0];
}

void solve()
{
	for (int i = 0; i < N; ++i)
	{
		solutia += (a[i].x * a[i + 1].y - a[i + 1].x * a[i].y);
	}
	solutia *= 0.5;
}

void write()
{
	ofstream fout("aria.out");
	fout << setprecision(5) << solutia << "\n";
	fout.close();
}

int main()
{
	read();
	solve();
	write();
	return 0;
}