3 0 0 1 0 0 1
4 1 0 0 1 -1 0 0 -1
0

0.5
2.0

#include<stdio.h>
#include<math.h>
struct point{
   double x,y;
}p[110];
double cross(point a,point b){
	return (a.x*b.y-a.y*b.x)/2.0;
}
int main(){
	int n;
	while(scanf("%d",&n),n){
		for(int i=0;i<n;++i){
			scanf("%lf%lf",&p[i].x,&p[i].y);
		}
		double res=0;
		for(int i=1;i<=n;++i){
			res+=cross(p[i%n],p[i-1]);
		}
		printf("%.1lf\n",fabs(res));
	}
	return 0;
}