/*
  AOJ 1100
  Title:
  @kankichi573
*/
#include <stdio.h>
int n,xx0,yy0,xx1,yy1,xx00,yy00;
main()
{
  double s;
  int i,cnt=0;
    
  while(scanf("%d",&n) && n)
    {
      cnt++;
      s=0.0;
      scanf("%d %d",&xx0,&yy0);
      xx00=xx0;yy00=yy0;
      for(i=1;i<n;i++)
	{
	  scanf("%d %d",&xx1,&yy1);
	  s+=(xx0-xx1)*(yy0+yy1)/2.0;
	  xx0=xx1;yy0=yy1;
	}
      s+=(xx0-xx00)*(yy0+yy00)/2.0;
      printf("%d %.1lf\n",cnt,-s);
    }
  return(0);
}