1 //任意多边形的面积
 2 #include <iostream>
 3 #include <cstring>
 4 #include <cstdio>
 5 #include <math.h>
 6 using namespace std;
 7 struct Node{
 8    int x,y;
 9 }c[105];
10 int main(){
11     int n,i;
12     double result;
13     //x[i]*y[i+1]-x[i+1]*y[i]的累加和
14     while(scanf("%d",&n)&&n!=0){
15         for(i=0;i<n;i++){
16             cin>>c[i].x>>c[i].y;
17         }
18         c[n].x=c[0].x;
19         c[n].y=c[0].y;
20         result=0;
21         for(i=0;i<n;i++)
22             result+=0.5*(c[i].x*c[i+1].y-c[i].y*c[i+1].x);
23  
24         printf("%.1lf\n",result);//顺时针输入的要取相反数.
25     }
26     return 0;
27 }