#include <fstream>
#include <iomanip>
#define Nmax 100005

using namespace std;

string file="aria";

ifstream f( (file + ".in").c_str() );
ofstream g( (file + ".out").c_str() );

struct punct{
double x, y;
}a[Nmax];

int n;

void aria()
{
    double s=0;
    for ( int i = 1; i <= n; i ++ )
    {
        s+=a[i].x*a[i+1].y-a[i].y*a[i+1].x;
    }
    s/=2;
    g << fixed << setprecision(5) << s;
}

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

    return 0;
}