#include<stdio.h>
#include<math.h>

int main(void){
  int count;
  int n,i;
  int x[50],y[50];
  double s;

  count=0;
  while(1){
    scanf("%d",&n);
    if(n==0)break;
    count++;
    s = 0;
    for(i=0;i<n;i++)scanf("%d %d",&x[i],&y[i]);

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

    printf("%d %1.1lf\n",count,s);
  }
  return 0;

}