#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
int n;
struct punct {
double x,y;}v[100005],x;
int crossproduct (punct a, punct o, punct b)
{
    int d;
    d= (a.x-o.x)*(b.y-o.y)-(b.x-o.x)* (a.y-o.y);
    return d;
}
int A;
int main()
{
    fin>>n;
    fin>>x.x>>x.y;
    fin>>v[1].x>>v[1].y;
    for (int i=3;i<=n;i++)
    {
        fin>>v[i].x>>v[i].y;
        A+=crossproduct(v[i],x,v[i-1]);
    }
    if (A<=0)
        fout << setprecision(5) << fixed <<A*-1;
    else
    fout<< setprecision(5) << fixed << A;
    return 0;
}