1 #include<iostream>
 2 #include<cstdio>
 3 #include<cmath>
 4 using namespace std;
 5 #define N 105
 6 struct point{
 7     double x ; double y ;
 8     point (double x_, double y_)
 9     {
10         x = x_;
11         y = y_;
12     }
13     point (){}
14     point operator - (const point a) const
15     {
16         return point(x-a.x,y-a.y);
17     }
18     double operator * (const point a) const
19     {
20         return x*a.y - y*a.x;
21     }
22 }P[N];
23 int main()
24 {
25     int n ;
26     while(~scanf("%d",&n),n)
27     {
28         for(int i = 0 ;i < n ; i++)
29         {
30             scanf("%lf%lf",&P[i].x,&P[i].y);
31         }
32         double area = 0;
33         point A = P[0];
34         for(int i = 0 ;i < n-1 ;i++)
35         {
36             point B = P[i];
37             point C = P[i+1];
38             area+=0.5*((B-A)*(C-A));
39         }
40         printf("%.1f\n",area);
41     }
42     return 0;
43 }