#include <fstream>

using namespace std;
fstream fin("poligon.in");
ofstream fout("poligon.out");

struct nod{float x,y; nod* urm;};

float arie(nod *prim)
{
    float aria=0;
    nod *p=prim;
    do
    {
        aria+=(p->x*p->urm->y - p->urm->x*p->y);
        p=p->urm;
    }while (p!=prim);
    return aria/2;
}
int main()
{
    int n, i;
    float x, y;
    nod *prim=new nod, *p=prim;
    fin>>n;
    fin>>x>>y;
    p->x=x;
    p->y=y;
    p->urm=prim;
    for (i=1; i<n; i++)
    {
        nod *temp=new nod;
        fin>>x>>y;
        temp->x=x;
        temp->y=y;
        temp->urm=prim;

        p->urm=temp;
        p=p->urm;
    }
    fout<<arie(prim);
    return 0;
}