#include <fstream>
#include <vector>
#include <iomanip>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n;
long double A;
vector <pair<double,double>> p;
long double det(double xa,double ya,double xb,double yb);
int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        double x,y;
        fin>>x>>y;
        p.push_back({x,y});
        if(i>1)
        {
            A+=det(p[p.size()-1].first,p[p.size()-1].second,p[p.size()-2].first,p[p.size()-2].second);
        }
    }
    A+=det(p[0].first,p[0].second,p[p.size()-1].first,p[p.size()-1].second);
    fout<<setprecision(6)<<fixed;
    fout<<A<<'\n';
    return 0;
}
long double det(double xa,double ya,double xb,double yb)
{
    long double aux=1.0*ya*xb-1.0*yb*xa;
    aux=aux/2.0;
    return aux;
}