#include <fstream>
#include <iomanip>

using namespace std;
int n;
double a;
double x[100006], y[100006];
double arie(double x1, double x2, double y1, double y2)
{
    return ((y1+y2)*(x2-x1))/2;
}
int main()
{
    ifstream in ("aria.in");
    ofstream out ("aria.out");
    in>>n;
    for(int i=0;i<n;i++)
    {
        in>>x[i]>>y[i];
    }
    x[n]=x[0], y[n]=y[0];
    for(int i=0;i<n;i++)
        a+=arie(x[i], x[i+1], y[i], y[i+1]);
    if(a<0)
        a*=(-1);
    out<<setprecision(20.5)<<a;
    return 0;
}