#include<fstream>
using namespace std;
struct pct{double x,y;}v[100003];
double rez;
int i,n;
double arie(pct A, pct B, pct C)
{double S=0;
  S=(A.x-B.x)*(B.y-C.y) - (B.x-C.x)*(A.y-B.y);
  if(S<0)S=-S;
  return S;
}
int main()
{
    ifstream f("aria.in");ofstream g("aria.out");
    f>>n;
    for(i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
    for(i=2;i<n;i++)
     rez += arie(v[1], v[i], v[i+1]);
     g<<rez/2;
    f.close();g.close();
    return 0;
}