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

typedef struct lol {
     double x,y;   
}troll;

int i,n;
long double rs;
troll a[100005];

int main()
{
  ifstream cin("aria.in");
  ofstream cout("aria.out");
  
  cin>>n;
  for(i=1;i<=n;i++) cin>>a[i].x>>a[i].y;
  
  for(i=1;i<=n;i++) rs+=a[i].x*a[i+1].y-a[i+1].x*a[i].y;
  rs+=a[n].x*a[1].y-a[1].x*a[n].y;
  rs/=2; rs=(rs<0) ? -rs:rs;
  
  cout<<setprecision(5);
  cout<<rs<<'\n';     
    
 return 0;   
}