#include <stdio.h>
#include <iostream>
#include <stack>
#include <vector>
#include <utility>
#include <list>
#include <algorithm>
#include <cmath>
using namespace std;

int main()
{
    int count = 1;
    while (true)
    {
        int n;
        cin >> n;
        if (n == 0)
            return 0;

        int x[n], y[n];
        cin >> x[0] >> y[0];
        for (int i = 1; i < n; i++)
        {
            cin >> x[i] >> y[i];
            x[i] -= x[0];
            y[i] -= y[0];
        }

        double S = 0;
        double tmp;
        for (int i = 2; i < n; i++)
        {
            S += x[i] * y[i - 1] - x[i - 1] * y[i];
        }
        S /= 2;
        printf("%d %.1lf\n", count++, S);
    }
}

