#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, *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)<<'\n';
    return 0;
}