/**
 * AOJ #1100: Area of Polygons (ICPC Domestic 1998)
 */

#include <stdio.h>

#define MAX_N 50

/**
 * Compute the area of a polygon with n vertices.
 * The coordinates in x[] and y[] are given in the clockwise order.
 * This function adopts the formula of the area using the vector product.
 */
double area(int n, int x[], int y[])
{
  double s = 0.0;
  int i, ix;
  for (i = 0; i < n; i++) {
    ix = (i + 1) % n;
    s += x[i] * y[ix] - x[ix] * y[i];
  }
  return 0.5 * (s < 0.0 ? -s : s);
}

int main()
{
  static int x[MAX_N], y[MAX_N];
  int n, i, cnt = 0;
  for (;;) {
    scanf("%d", &n);
    if (n == 0) break;
    for (i = 0; i < n; i++) {
      scanf("%d%d", &x[i], &y[i]);
    }
    printf("%d %.1f\n", ++cnt, area(n, x, y));
  }
  return 0;
}