E - 改革春风吹满地

按顺时针或者逆时针顺序输入n个点，求输入点围城的多边形的面积。凸凹都可以计算。

模板

 #include <iostream>
 #include <cstring>
 #include <string>
 #include <map>
 #include <set>
 #include <algorithm>
 #include <fstream>
 #include <cstdio>
 #include <cmath>
 #include <stack>
 #include <queue>
 using namespace std;
 const double Pi=3.14159265358979323846;
 typedef long long ll;
 const int MAXN=+;
 const int dx[]={,,,,-};
 const int dy[]={,-,,,};
 const int INF = 0x3f3f3f3f;
 const int NINF = 0xc0c0c0c0;
 const ll mod=1e9+;
 struct point{
     double x;double y;
     point (double xx,double yy):x(xx),y(yy) {}
 };

 double ComAre(const vector <point> &V)
 {
     int len=V.size();
     if(len<) return 0.0;
     double s=;
     for(int i=;i<len;i++)
     {
         s+=V[i].x*V[(i+)%len].y-V[i].y*V[(i+)%len].x;
     }
     return fabs(s/);
 }

 int main()
 {
     int n;
     while(cin>>n&&n)
     {
         vector <point> V;
         for(int i=;i<=n;i++)
         {
             double a,b;cin>>a>>b;
             V.push_back(point(a,b));
         }
         double ans=ComAre(V);
         printf("%.1lf\n",ans);
     }
     return ;
  }