#include <fstream>
#include <iomanip>
#include <cmath>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int n;
long double r;
struct point
{ long double x,y;
}p[100001];
int arie(point a,point b,point c)
{ a.x-=c.x;
  a.y-=c.y;
  b.x-=c.x;
  b.y-=c.y;
  return fabs((long double)(a.x*b.y-b.x*a.y)/2);
}
int main()
{ in>>n;
  for(int i=1;i<=n;i++)
    in>>p[i].x>>p[i].y;
  for(int i=1;i<=n-2;i++)
    r+=arie(p[i],p[i+1],p[i+2]);
  out<<setprecision(6)<<fixed<<r;
  in.close();
  out.close();
  return 0;
}