# include <fstream>
# include <iomanip>
# include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int i,j,n;
long double var,S;
struct elem
{
    long double x,y;
}a[100005];
int main ()
{
    f>>n;
    if (n<=2)
    {
        g<<"0\n";
        return 0;
    }
    for (i=1; i<=n; ++i)
        f>>a[i].x>>a[i].y;
    for (i=3; i<=n; ++i)
    {
        var=abs(a[1].x*a[i-1].y+a[i-1].x*a[i].y+a[i].x*a[1].y-a[1].x*a[i].y-a[i].x*a[i-1].y-a[i-1].x*a[1].y)/2;
        S+=var;
    }
    g<<fixed<<setprecision(5)<<S<<"\n";
    return 0;
}