#include<iostream>
#include<cstdio>
#include<cmath>
#include<complex>
#include<vector>
#include<queue>
using namespace std;
typedef complex<int> xy_t;
int main()
{
  int n,k = 1;
  double x,y,sum = 0.0;
  queue<double> answer;
  while(cin >> n && n > 0){
    vector<xy_t> p(n);
    for( int i = 0; i < n; ++i){
      cin >> x >> y;
      p[i] = xy_t(x,y);
    }
    for( int i = 0; i + 2 < n; ++i){
      xy_t a = p[0],b = p[i + 1],c = p[i + 2];
      sum += ((conj(b - a)*(c - a)).imag())/2.0;
    }
    answer.push(abs(sum));
    sum = 0.0;
  }
  while(!answer.empty()){
    printf("%d %.1f\n",k,answer.front());
    answer.pop();
    ++k;
  }
}