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

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

struct point { int x, y; };

point puncte[100000];
int N;
double solutia;

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

double aria(point A, point B, point C)
{
	int first = A.x * B.y + B.x + C.y + C.x * A.y;
	int second = C.x * B.y + A.x * C.y + B.x * A.y;
	return (first - second) / 2.;
}

void solve()
{

	for (int i = 2; i < N; ++i)
	{
		solutia += aria(puncte[0], puncte[i - 1], puncte[i]);
	}
}

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

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