#include <stdio.h>

int main(void){
  int n;
  int p[50][2];
  int i;
  int loop=1;
  double area;

  while( 1 ){
    scanf("%d ",&n);
    if( !n )
      break;

    area = 0.0;
    for(i=0;i<n;i++){
      scanf("%d %d ",&p[i][0],&p[i][1]);
    }
    for(i=0;i<n-1;i++)
      area = area+(p[i][0]-p[i+1][0])*(p[i][1]+p[i+1][1]);
    area = area+(p[n-1][0]-p[0][0])*(p[n-1][1]+p[0][1]);

    area = area/2.0;
    if( area<0 ) area=-area;

    printf("%d %.1f\n",loop++,area);
  }

  return 0;
}