#include<iostream>
#include<cmath>
#include<cstdio>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; ++i)

int x[100], y[100];

int main() {
  int n;
  for (int ii = 1;; ++ii) {
    cin >> n;
    if (n == 0) break;
    double res = 0;
    rep (i, n) cin >> x[i] >> y[i];
    rep (i, n) res += x[i] * y[(i + 1) % n] - y[i] * x[(i + 1) % n];
    printf("%d %.1lf\n", ii, abs(res / 2)); 
  }
}