#include <fstream>
#include <iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");

int n,i;
long double aria(0),X[100000],Y[100000];
int main()
{
  cin>>n;
  for (i=1;i<=n;++i) cin>>X[i]>>Y[i];
  aria+=(X[n]*Y[1]-X[1]*Y[n]);
  for (i=1;i<=n;++i) aria+=(X[i]*Y[i+1]-X[i+1]*Y[i]);
  aria=aria/2;
  cout<<fixed<<setprecision(5)<<aria; 
return 0;
}