#include <stdio.h>

typedef struct {
	double x, y;
} Coord;

double area_poly( Coord *coord, int n ) {
	int i;
	double sum = 0.0;

	for ( i = 1; i <= n; i++ )
		sum += coord[ i - 1 ].x * coord[ i % n ].y - coord[ i - 1 ].y * coord[ i % n ].x;

	return -sum / 2.0;
}

int main( void ) {
	int i;
	int n, cnt;
	Coord coord[ 100 ];

	cnt = 0;
	while ( scanf( "%d", &n ), n ) {
		for ( i = 0; i < n; i++ )
			scanf( "%lf %lf", &coord[ i ].x, &coord[ i ].y );

		printf( "%d %.1f\n", ++cnt, area_poly( coord, n ) );
	}

	return 0;
}