#include<fstream>
#include<math.h>
using namespace std;

#define MAX 100002

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

int N;
struct POINT
{
	double x, y;
};

POINT  P[MAX];


int main()
{
	double A = 0;
	in >> N;
	for (int i = 1;i <= N;++i)
	{
		in >> P[i].x >> P[i].y;
	}
	P[N + 1] = P[1];
	for (int i = 1;i <= N;++i)
	{
		A += (P[i].x*P[i+1].y - P[i+1].x*P[i].y)/2;
	}
	out.precision(5);
	out <<A;
	return 0;
}