#include<bits/stdc++.h>
#define MOD 1000000007
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3f
#define EPS (1e-10)
#define rep(i,n)for(int i=0;i<(n);i++)
using namespace std;
typedef long long ll;
typedef pair<int, int>P;

int x[50], y[50];
int main() {
	int n, cnt = 1;
	while (scanf("%d", &n), n) {
		rep(i, n)scanf("%d%d", &x[i], &y[i]);
		int sum = 0;
		rep(i, n)sum += x[i] * y[(i + 1) % n] - y[i] * x[(i + 1) % n];
		printf("%d %.1lf\n", cnt, abs(sum) / 2.);
		cnt++;
	}
}