#include <iostream>
#include <cmath>
#include <iomanip>
#include <fstream>
using namespace std;
/*ifstream fin("ariatriunghi.in");
ofstream fout("ariatriunghi.out");*/
ifstream fin("aria.in");
ofstream fout("aria.out");
/*struct elev
{
    char nume;//initiala; campul nume
    float nota_mate,nota_info;//campul nota_mate, campul nota_info
    int varsta;// campul varsta
};
elev ion;*///S-a declarat o variabila ion de tip elev

/*struct punct
{
    float x,y;
};
punct a,b,c;
float det,mod_a,mod_b;*/
struct punct
{
    float x,y;
};
punct v[100001];
float arie;
int n,i;
float arietriunghi(punct a,punct b,punct c)
{
    float det=a.x*(b.y-c.y)-b.x*(a.y-c.y)+c.x*(a.y-b.y);
    return abs(det)/2;
}
int main()
{
    /*cin>>ion.varsta;// Am accesat campul varsta al variabilei ion
    cin>>ion.nota_mate>>ion.nota_info;
    cout<<ion.varsta<<" ";
    cout<<ion.nota_info<<" "<<ion.nota_mate;*/

    //Afisare ariei unui triunghi in functie de coordonatele x si y ale varfurilor
    /*cin>>a.x>>a.y>>b.x>>b.y>>c.x>>c.y;
    det=a.x*b.y+b.x*c.y+c.x*a.y-c.x*b.y-a.y*b.x-a.x*c.y;
    det=a.x*(b.y-c.y)+b.x*(c.y-a.y)+c.x*(a.y-b.y);

    cout<<abs(det)/2;*/

    //Problema 4
    /*cin>>a.x>>a.y>>b.x>>b.y;
    if(a.x!=b.x||a.y!=b.y)
    {
        mod_a=sqrt(a.x*a.x+a.y*a.y);
        cout<<mod_a<<'\n';
        mod_b=sqrt(b.x*b.x+b.y*b.y);
        cout<<mod_b<<'\n';
        cout<<(a.x*b.x+a.y*b.y)/(mod_a*mod_b);
    }
    else
        cout<<1;*/

        //Aria triunghi-#928 pbinfo
        /*struct punct
        {
            float x,y;
        };
        punct a,b,c;
        float det,mod_a,mod_b;
    fin>>a.x>>a.y>>b.x>>b.y>>c.x>>c.y;
    det=a.x*b.y+c.x*a.y+b.x*c.y-c.x*b.y-
    b.x*a.y-a.x*c.y;
    det=a.x*(b.y-c.y)-b.x*(a.y-c.y)+c.x*(a.y-b.y);
    fout<<fixed<<setprecision(1)<<abs(det)/2;*/

    //Aria unui triunghi oarecare in functie de laturi
    /*int a,b,c;
    cin>>a>>b>>c;
    int p=(a+b+c)/2;
    cout<<double(sqrt(p*(p-a)*(p-b)*(p-c)));*/
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    for(i=1;i<=n-2;i++)
        arie+=arietriunghi(v[1],v[i+1],v[i+2]);
    fout<<arie;
    return 0;
}