#include <fstream>
#include <cmath>
#include <iomanip>
#define NMAX 100001

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

int i, n;
double rez=0;

struct punct
{
    double x, y;
};
punct v[NMAX];

int main()
{
    f>>n;
    for (i=1; i<=n; ++i)
        f>>v[i].x>>v[i].y;

    v[n+1]=v[1];

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

    g<<fixed<<setprecision(5)<<fabs(rez)/2<<'\n';
    return 0;
}