#include <iomanip>
#include <fstream>
#include <cmath>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct Punct
{ double x,y;
};
double AriaPoligon(int n, Punct p[])
{
    p[n+1] = p[1];
    int i;
    double s = 0;
    for (i = 1; i <= n; i++)
        s += p[i].x * p[i+1].y - p[i].y * p[i+1].x;
    return fabs(s/2);
}
 Punct a[1000000];
int main()
{
	int n;
	fin>>n;
	for(int i=1; i<=n; i++)
		fin>>a[i].x>>a[i].y;
fout<<setprecision(6)<<fixed<<AriaPoligon(n,a);
}