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

using namespace std;
int n;
typedef struct { double x, y;} punct;


double arietr(punct A, punct B, punct C)
{
  return abs((B.x-A.x)*(C.y-A.y) - (C.x-A.x) * (B.y-A.y)) /2 ;
}
double arie (punct p[100002])
{
    int i;
   double ar=0;
    for (i=2; i<n; ++i)
{
     ar+=arietr(p[1],p[i],p[i+1]);
}
    return ar;
}
int main()
{
   ifstream f("aria.in");
   ofstream g("aria.out");
   punct A[100002];
   punct q, w, e;
   int i;
   f >> n;

   for (i=1; i<=n; ++i)
   {
       f >> A[i].x >> A[i].y;
   }

   g << setprecision(10) << arie(A) << "\n";
    return 0;
}