poj 3970 Party

#include<iostream>        //求任意多边形面积
#include<cmath>
using namespace std;
struct point
{
    double x,y;
}p[1000];
int main()
{
    int n;
    while(cin>>n,n)
    {
        for(int i=0;i<n;++i)
            cin>>p[i].x>>p[i].y;
        if(n<3)
        {
            printf("0\n");
            continue;
        }
        double area=0;
        for(int i=0;i<n;++i)
            area+=p[i].x*p[(i+1)%n].y-p[i].y*p[(i+1)%n].x;
        printf("%.f\n",fabs(area)/2);
    }
    return 0;
}