#include <fstream>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
///punctele sunt date in ordine trigonometrica
long double det(int x1,int y1, int x2, int y2)
{
    return (x1*y2-x2*y1);
}
int main()
{
    int n;
    long double x1,y1,cx1,cy1,x2,y2,arie=0;
    f>>n;
    f>>x1>>y1;
    cx1=x1,cy1=y1;
    for(int i=2;i<=n;++i)
    {
        f>>x2>>y2;
        arie=arie+det(x1,y1,x2,y2);
        x1=x2,y1=y2;
    }
    arie=arie+det(x1,y1,cx1,cy1);
    g<<arie/2.0;
    f.close();
    g.close();
    return 0;
}