#include<stdio.h>
struct V{double x,y;}v[50],a,b;
V operator-(V a,V b){V c={a.x-b.x,a.y-b.y};return c;}
int main()
{
	int n,i,c=0;
	double s;
	while(scanf("%d",&n),n)
	{
		for(s=i=0;i<n;++i)scanf("%lf%lf",&v[i].x,&v[i].y);
		for(i=2;i<n;++i)
		{
			a=v[i]-v[0],b=v[i-1]-v[0];
			s+=(a.x*b.y-a.y*b.x)/2;
		}
		printf("%d %.1f\n",++c,s);
	}
	return 0;
}