x0,y0,x,y,v,w,S;
main(i,n){
	for(;scanf("%d",&n)*n;){
		S=0;
		scanf("%d%d",&x0,&y0);
		v=x0,w=y0;
		for(;--n;){
			scanf("%d%d",&x,&y);
			S+=(x-v)*(y+w);
			v=x,w=y;
		}
		S+=(x0-v)*(y0+w);
		printf("%d %.1f\n",i++,S/2.);
	}
}