//45
#include<iostream>
#include<cmath>
#include<iomanip>
#include<complex>

using namespace std;

int main(){
  int no=0;
  for(int n;cin>>n,n;){
    typedef complex<double> P;
    int x,xx,y,yy;
    cin>>x>>y>>xx>>yy;
    P p(x,y);
    P l=P(xx,yy)-p;
    double area=0;
    for(int i=0;i<n-2;i++){
      int x,y;
      cin>>x>>y;
      P ll=P(x,y)-p;
      area+=-(l.real()*ll.imag()-l.imag()*ll.real())/2;
      l=ll;
    }
    no++;
    cout<<no<<' '<<fixed<<setprecision(1)<<area<<endl;
  }
  return 0;
}