#include <fstream>

using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");

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

float calculezaria (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;
    float x, y;
    nod * prim= new nod, * p =prim;
    fin>>n;
    fin>>x>>y;
    p->x=x;
    p->y=y;
    p->urm= prim;
    for(int 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<< calculezaria(prim);
    return 0;
}