#include <fstream>
#include <iomanip>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

double det(pair<double,double> x, pair<double,double> y)
{
    return x.first * y.second - x.second * y.first;
}

pair<double,double> x,y,aux;
int n,i;
double Aria;

int main()
{
    fin>>n;
    fin>>x.first>>x.second;
    aux = x;
    for (i=2;i<=n;i++)
    {
        fin>>y.first>>y.second;
        Aria += det(x,y);
        x = y;
    }
    Aria += det(x,aux);
    Aria /= 2;
    fout<<fixed;
    fout<<setprecision(5)<<Aria;
    return 0;
}