#include <fstream>
#include <iomanip>
#include <cmath>

using namespace std;

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

int n;
double AriaPoligon;

struct punct {
int x, y;
}v[100015];

void read ()
{
 int i, xx, yy;
  f >> n;
   for (i=1; i<=n; i++)
   {
       f >> xx >> yy;
       v[i].x = xx;
       v[i].y = yy;
   }
}

double aria (int x, int y, int x2, int y2, int x3, int y3)
{
  double rez;
   rez = fabs(x * y2 + x2 * y3 + x3 * y - x3 * y2 - x * y3 - x2 * y);
   rez /= 2;
  return rez;
}

void solve ()
{
  int i, xx, yy, x2, y2, x3, y3;
   xx = v[1].x, yy = v[1].y;
    for (i=3; i<=n; i++)
    {
        x2 = v[i-1].x;
        y2 = v[i-1].y;
        x3 = v[i].x;
        y3 = v[i].y;
        AriaPoligon += aria(xx, yy, x2, y2, x3, y3);
    }
  g << fixed << setprecision(6) << AriaPoligon;
}

int main()
{
read();
solve();
    return 0;
}