#include <fstream>
#include <iomanip>
using namespace std;

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

int n;
double a;
struct st{double x,y;}v[100003];

int main()
{
      f>>n;
      for(int i=1;i<=n;++i) f>>v[i].x>>v[i].y;
      v[n+1]=v[1];
      for(int i=1;i<=n;++i) a+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2;
      if(a<0) a=-a;
      g<<fixed<<setprecision(5)<<a;
      return 0;
}