#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
 
#define loop(n, i) for(int i=0;i<n;i++)
#define loop_from_to(from, to, i) for(int i=from;i<=to;i++)
#define repeat(n) for(int i__=0;i__<n;i__++)
#define multi_cin(n, array) loop(n,i){cin>>array[i];}
#define assign_if_smaller(var, val) if(var>val)var=val;
#define assign_if_larger(var, val) if(var<val)var=val;
#define each(itr, c) for(__typeof((c).begin()) itr=(c).begin(); itr!=(c).end(); itr++)
#define r_each(itr, c) for(__typeof((c).rbegin()) itr=(c).rbegin(); itr!=(c).rend(); itr++)

using namespace std;

double calc(int x1,int y1,int x2,int y2) {
  return (double)(x2-x1)*(y1+y2)/2;
}

int main(int argc, char const* argv[])
{
  int count=0;
  while (true) {
    int n;cin>>n;
    if(!n) break;
    int px,py;cin>>px>>py;
    int xzero=px,yzero=py;
    double area=0;
    n--;
    while(n--) {
      int x,y;cin>>x>>y;
      area+=calc(px,py,x,y);
      px=x;py=y;
    }
    area+=calc(px,py,xzero,yzero);
    cout<<++count<<" ";
    cout<<fixed<<setprecision(1)<<area<<endl;
  }
  return 0;
}