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


int main(){
	int cnt=1;
	int n;
	while(cin>>n,n){
		double ans=0.0;
		vector<int>x(n+1),y(n+1);
		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++){
			ans+=(x[i]*y[i+1]-y[i]*x[i+1])*0.5;
		}
		
		
		printf("%d %.1f\n",cnt++,fabs(ans));
	}
	return 0;
}
