#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
double aria (double xb,double yb,double xc,double yc)
{
    return yc*xb-yb*xc;
}
int n,i,j;
double x[100001],y[100001],sum;
int main()
{
    fin>>n;
    for (i=1; i<=n; i++)
        fin>>x[i]>>y[i];
    for (i=1; i<=n; i++)
    {
        j=i+1;
        if (j>n)
            j=1;
        sum+=aria (x[i],y[i],x[j],y[j]);
    }
    sum/=2;
    fout<<fixed<<setprecision (6)<<sum;
    return 0;
}