#include <stdio.h>


int main () {
    int n, x[50], y[50], i, seq = 0;

    while ((scanf("%d", &n)) != EOF && n) {
        double area = .0;
        int j;
        for (i = 0; i < n; i++) {
            scanf("%d%d", &x[i], &y[i]);
        }

        for (i = 0; i < n; i++) {
            j = (i + n -1) % n;
            area += (x[i] - x[j]) * (y[i] + y[j]);
        }

        printf("%d %.1f\n", ++seq, area * .5);
    }

    return 0;
}