#include <fstream>
#include <iomanip>
using namespace std;
ifstream f ("aria.in");
ofstream g ("aria.out");
int x[100004],y[100004],i,n;
double a,a1;
int main()
{
    f>>n;
    for (i=1;i<=n;i++)
        f>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    a=0.0;
    for (i=1;i<=n;i++)
    {
        a1=(1.0*x[1]*y[2]-1.0*y[1]*x[2])/2.0;
        a=0.0+a+a1;
    }
    g<<setprecision(5)<<fixed<<a;
    return 0;
}