#include <iostream>
#include <stdio.h>
using namespace std;

int main(){
	int n, x[51], y[51], c = 1;
	double pori;
	while(cin >> n, n != 0){
		pori = 0;
		for(int i = 0; i < n; i++){
			cin >> x[i] >> y[i];
		}
		x[n] = x[0];
		y[n] = y[0];
		for(int i = 0; i < n; i++){
			pori += (double)(y[i] + y[i+1]) * (-x[i] + x[i+1]) / 2.0;
		}
		printf("%d %.1f\n", c, pori);
c++;
	}
	return 0;
}