#include <fstream>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct nod
{
    float x, y;
    nod *urm;
};
float calculeazaAria(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;
    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<<calculeazaAria(prim);
    return 0;
}