# include <fstream>
# include <algorithm>
# include <iomanip>
# define NR 100005
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int i,j,n;
double aria, x[NR], y[NR];
int main ()
{
    f>>n;
    for (i=1; i<=n; ++i)
        f>>x[i]>>y[i];
    x[n+1]=x[1]; y[n+1]=y[1];

    for (i=1; i<=n; ++i)
        aria=aria + (x[i]*y[i+1] - x[i+1]*y[i]);

    g<<fixed<<setprecision(5)<<aria/2<<"\n";

    return 0;
}