#include <stdio.h>

int main(void){
	int n,x[100],y[100];
	int va_x,va_y,vb_x,vb_y;
	int i,count=1;
	double s=0.0,sum=0.0;

	scanf("%d",&n);

	do{
		sum = 0.0;
		for(i = 0;i < n;i++){
			scanf("%d %d",&x[i],&y[i]);
		}

		for(i = 1;i < n-1;i++){
			va_x = x[i]-x[0];
			va_y = y[i]-y[0];
			vb_x = x[i+1]-x[0];
			vb_y = y[i+1]-y[0];
			
			s = ((double)va_x*vb_y-(double)va_y*vb_x)/2;
			if(s >= 0){
				sum+=s;
			} else if(s < 0){
				s=-s;
				sum+=s;
			}
			printf("i = %d i+1 = %d\n",i,i+1);
			printf("s = %f sum = %f\n",s,sum);
		}

		printf("%d %.1f\n\n",count++,sum);
		scanf("%d",&n);
	}while(n != 0);

	return 0;
}