// Aizu 1100: Area of Polygons
// 2017.9.12 bal4u@uu

#include <stdio.h>
#include <math.h>

typedef struct { int x, y; } PP;
PP p[52];

double calc_area(int n, PP *pp)
{
	int i;
	double s;

	if (n < 3) return 0;
	for (s = 0, i = 0; i < n; i++)
		s += (double)(pp[i].x-pp[i+1].x) * (double)(pp[i].y+pp[i+1].y);
   return fabs(s)/2.0;
}

int main()
{
	int n, i, cno = 0;

	while (scanf("%d", &n) && n) {
		for (i = 0; i < n; i++) scanf("%d%d", &p[i].x, &p[i].y); p[n] = p[0];
		printf("%d %.1lf\n", ++cno, calc_area(n, p));
	}
	return 0;
}