#include<fstream>
#include<iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
long double x1,y1,x2,y2,arie,xprim,yprim;
int main()
{
    int i,n;
    f>>n>>x1>>y1;
    xprim=x1;yprim=y1;
    for(i=2;i<=n;i++)
    {
        f>>x2>>y2;
        arie=arie+0.5*(y1+y2)*(x2-x1);
        x1=x2;
        y1=y2;
    }
    arie+=0.5*(yprim+y1)*(xprim-x1);
    if(arie<0)arie=(-1)*arie;
    g<<fixed<<setprecision(6)<<arie;
    return 0;
}