1 /*************************************************************************
 2     > File Name: code/hdoj/2036.cpp
 3     > Author: 111qqz
 4     > Email: rkz2013@126.com 
 5     > Created Time: 2015年11月06日 星期五 12时05分30秒
 6  ************************************************************************/
 7 
 8 #include<iostream>
 9 #include<iomanip>
10 #include<cstdio>
11 #include<algorithm>
12 #include<cmath>
13 #include<cstring>
14 #include<string>
15 #include<map>
16 #include<set>
17 #include<queue>
18 #include<vector>
19 #include<stack>
20 #include<cctype>
21 #define fst first              
22 #define lson l,m,rt<<1
23 #define rson m+1,r,rt<<1|1
24 #define ms(a,x) memset(a,x,sizeof(a))
25 using namespace std;
26 const int dx4[4]={1,0,0,-1};
27 const int dy4[4]={0,-1,1,0};
28 typedef long long LL;
29 #define sec second
30 const int inf = 0x3f3f3f3f;
31 const int N=105;
32 int n;
33 struct point 
34 {
35     double x,y;
36     point(){}
37     point(double _x,double _y):
38     x(_x),y(-y){};
39     void input()
40     {
41     scanf("%lf%lf",&x,&y);
42     }
43     double det(point p)
44     {
45     return x*p.y-y*p.x;
46     }
47 };
48 struct  polygon
49 {
50     point  p[N];
51     void input()
52     {
53     for ( int i = 0 ; i < n ; i++)
54     {
55         p[i].input();
56     }
57     }
58     double getarea()
59     {
60     double sum= 0 ;
61     int i;
62     for ( int i = 0 ; i < n ; i++ )
63     {
64         sum+=p[i].det(p[(i+1)%n]);
65     }
66     return fabs(sum)/2;
67     }
68 }pol;
69 int main()
70 {
71   #ifndef  ONLINE_JUDGE 
72    freopen("in.txt","r",stdin);
73   #endif
74    while (scanf("%d",&n)!=EOF&&n)
75     {
76     pol.input();
77     printf("%.1f\n",pol.getarea());
78     }
79 
80   
81    
82  #ifndef ONLINE_JUDGE  
83   fclose(stdin);
84   #endif
85     return 0;
86 }