1 #include <iostream>
2 #include <cstdio>
3 using namespace std;
4
5 struct point
6 {
7 double x,y;
8 }p[110];
9
10 double siz(point p1,point p2,point p3)
11 {
12 return ((p1.x-p2.x)*(p1.y-p3.y)-(p1.x-p3.x)*(p1.y-p2.y))/2;
13 }
14
15 int main()
16 {
17 int n;
18 while(scanf("%d",&n)!=EOF)
19 {
20 if(n==0)
21 break;
22 for(int i=0;i<n;i++)
23 {
24 scanf("%lf%lf",&p[i].x,&p[i].y);
25 }
26 double ans=0;
27 for(int i=0;i<n;i++)
28 {
29 ans+=siz(p[0],p[i],p[(i+1)%n]);
30 }
31 printf("%.1lf\n",ans);
32 }
33 return 0;
34 }
