#include <iostream>
#include <stdio.h>

using namespace std;

int main(void){
  int N = 0;
  while(1){
    N++;
    int n;
    cin >> n;
    if(n == 0){return 0;}

    double x[n + 1];
    double y[n + 1];

    for(int i = 0 ; i < n ; i++){
      cin >> x[i];
      cin >> y[i];
    }
    x[n] = x[0];
    y[n] = y[0];

    double S = 0;

    for(int i = 0 ; i < n ; i++){
      S -= (x[i] * y[i + 1] - x[i + 1] * y[i]) / 2.0;
    }

    printf("%d %.1lf\n", N, S);
  }
}