
int main(void){
  int i,n;
  float ans,bx,by,x0,y0,x,y;

  for(i=1;;i++){
    scanf("%d",&n);
    if(!n) break;

    ans = 0;
    scanf("%f%f",&x0,&y0);
    bx=x0,by=y0;
    for(n--;n--;){
      scanf("%f%f",&x,&y);
      ans += bx*y - by*x;
      bx=x,by=y;
    }
    ans += x*y0 - y*x0;

    printf("%d %.1f\n",i,-ans/2);
  }

  return 0;
}