#include <fstream>
#include <cmath>
using namespace std;
const char iname[] = "aria.in";
const char oname[] = "aria.out";
ifstream fin(iname);
ofstream fout(oname);
int N, i;
double Arie = 0.0;
struct p{
	int x, y;
}P[100004];
int main()
{
	fin >> N;
	for (i = 1; i <= N; ++i) fin >> P[i].x >> P[i].y;
	P[N + 1] = P[1];
	for (i = 1; i <= N; ++i)
		Arie += (P[i].x * P[i + 1].y - P[i + 1].x * P[i].y);
	Arie /= 2.0;
	fout << fabs(Arie) << '\n';
	return 0;
}