#include <bits/stdc++.h>
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
const int Nmax =  100005;
struct Point
{
    double x , y;
};
Point a[Nmax];
int main()
{
    /**
        Alegem un punct oarecare in sistemul de coordonate XOY , fie el P(0 , 0)
        si formam n triunghiuri (PA1A2 , PA2A3 ... PA(n - 1)A(n) , PA(n)A1)
        Solutia : suma ariilor celor n triunghiuri = 1 / 2 * d , d =
                    | 0 , 0 ,   1 |
                    | xk , yk , 1 |
                    | xp , yp , 1 |
        Fara modul deoarece ariile negative vor anula zonele corespunzatoare ariilor pozitive care nu se afla
        in poligon
    */
    int n;
     double ar = 0;
    fin >> n;
    for(int i = 1 ; i <= n ; i++)
        fin >> a[i] . x >> a[i] . y;
    a[n + 1] = a[1];
    for(int i = 1 ; i <= n ; i++)
        ar += (a[i] . x * a[i + 1] . y - a[i + 1] . x * a[i] . y);
    ar = (double) ar / 2;
    fout << fixed << setprecision(7) << ar << "\n";
    fin.close();
    fout.close();
    return 0;
}