#include <math.h>
#include <stdio.h>
#pragma warning(disable : 4996)
int n, c, x[100], y[100];
double s(int xa, int ya, int xb, int yb, int xc, int yc) {
	return abs((xb - xa) * (yc - ya) - (xc - xa) * (yb - ya)) * 0.5;
}
int main() {
	while (true) {
		scanf("%d", &n);
		if (n == 0) break;
		for (int i = 0; i < n; i++) scanf("%d%d", &x[i], &y[i]);
		double ret = 0;
		for (int i = 2; i < n; i++) {
			ret += s(x[0], y[0], x[1], y[1], x[i], y[i]);
		}
		printf("%d %.1f\n", ++c, ret);
	}
	return 0;
}