求多边形面积，以此作为模板：


#include <iostream>
#include <cstdio>
using namespace std;
struct point{
  int x,y;
  point operator -(const point &b)
  {
      return (point){this->x - b.x,this->y - b.y};
  }
}p[102];
int crossmul(point a,point b)
{
    return a.x*b.y-a.y*b.x;
}
int main()
{
    int i,n;
    while(~scanf("%d",&n)&&n)
    {

        for(i=0;i<n;i++)
        {
            scanf("%d%d",&p[i].x,&p[i].y);
        }
        if(n<3){printf("0.0\n");continue;}
        int area2=0;
        for(i=1;i<n-1;i++)
            area2+=crossmul(p[i]-p[0],p[i+1]-p[0]);
        if(area2<0)
            area2=-area2;
        printf("%.1lf\n",1.0*area2/2);
    }
    return 0;
}