求多边形面积 HDU2036

#include <iostream>
#include <cstdio>
using namespace std;

struct point
{
    double x,y;
}p[110];

double siz(point p1,point p2,point p3)
{
    return ((p1.x-p2.x)*(p1.y-p3.y)-(p1.x-p3.x)*(p1.y-p2.y))/2;
}

int main()
{
    int n;
    while(scanf("%d",&n)!=EOF)
    {
        if(n==0)
            break;
        for(int i=0;i<n;i++)
        {
            scanf("%lf%lf",&p[i].x,&p[i].y);
        }
        double ans=0;
        for(int i=0;i<n;i++)
        {
            ans+=siz(p[0],p[i],p[(i+1)%n]);
        }
        printf("%.1lf\n",ans);
    }
    return 0;
}