#include<fstream>
#include<iomanip>
 using namespace std;
 ifstream f("aria.in");
ofstream g("aria.out");
 struct point
{
    double x,y;
};
 double area = 0;
point p[100001];
int n;
 int main()
{
    f>>n;
    for (int i=0;i<n;i++)
        f>>p[i].x>>p[i].y;
    p[n] = p[0];
    f.close();
  for (int i=0;i<n;i++)
    area += ( (p[i].x*p[i+1].y) - (p[i+1].x*p[i].y) );
    g<<setprecision(5)<<fixed<<area/2.0;
    g.close();
    return 0;
}