#include <stdio.h>

int main(void)
{
    int seq = 1;
    while(1){
        int n;
        int x[50], y[50];

        scanf("%d", &n);
        if(n == 0)
            break;

        int i;
        for(i = 0; i < n; i++){
            scanf("%d %d", &(x[i]), &(y[i]));
        }

        double area = 0;

        for(i = 0; i < n; i++){
            double temp = 0.5 * (((double)x[i] * (double)y[(i + 1)%n]) - ((double)x[(i + 1)%n] * (double)y[i]));
            area += temp;
        }

        if(area < 0)
            area *= -1;

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

    return 0;
}