#include<stdio.h>
#include<math.h>
int main(){
  int n,i,ax,ay,bx,by,x,y;
  double s;
  for(i=1;scanf("%d",&n),n;i++){
    scanf("%d %d %d %d",&ax,&ay,&bx,&by);
    s=0;
    for(n-=2;n;n--){
      scanf("%d %d",&x,&y);
      s+=((bx-ax)*(y-ay)-(by-ay)*(x-ax))/2.0;
      bx=x;
      by=y;
    }
    printf("%d %.1f\n",i,fabs(s));
  }
  return 0;
}