#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;

int n;
int x[51], y[51];

int main() {
	int caseId = 0;
	
	while (cin >> n) {
		if (n == 0) break;
		caseId++;
		int i;
		for (i = 0; i < n; i++) cin >> x[i] >> y[i];
		x[n] = x[0]; y[n] = y[0];
		
		int area = 0;
		for (i = 0; i < n; i++) {
			area += x[i] * y[i + 1] - y[i] * x[i + 1];
		}
		printf("%d %.1f\n", caseId, abs(area) * 0.5);
	}
	return 0;
}