#include <iostream>
#include <fstream>
#include <iomanip>
#define NMAX 100000
#define f first
#define s second
#define ld long double

using namespace std;

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

int n;
pair <ld, ld> v[NMAX+10];
ld A;

int main()
{
	fin >> n;
	for(int i=1; i<=n; i++)
		fin >> v[i].f >> v[i].s;
	v[n+1] = v[1];
	ld A = 0;
	for(int i=1; i<=n; i++)
		A = A + (v[i].f * v[i+1].s - v[i].s * v[i+1].f);
	fout << setprecision(12) << fixed << A * 0.5 << '\n';
	return 0;
}